diff --git a/src/controller/KissCount.cpp b/src/controller/KissCount.cpp
index 129054a..440b559 100644
--- a/src/controller/KissCount.cpp
+++ b/src/controller/KissCount.cpp
@@ -36,12 +36,12 @@ std::list<wxString> KissCount::GetUsers()
   return _db->GetUsers();
 }
 
-bool KissCount::IsValidUser(wxString user, wxString password)
+bool KissCount::IsValidUser(const wxString& user, const wxString& password)
 {
   return _db->IsValidUser(user, password) ;
 }
 
-void KissCount::LoadUser(wxString user)
+void KissCount::LoadUser(const wxString& user)
 {
   if (_user) delete _user;
   _user = _db->LoadUser(user) ;
@@ -66,22 +66,22 @@ User* KissCount::GetUser()
   return _user;
 }
 
-double KissCount::GetAccountAmount(wxString id, int month, int year)
+double KissCount::GetAccountAmount(const wxString& id, int month, int year)
 {
   return _db->GetAccountAmount(id, month, year);
 }
 
-void KissCount::UpdateOperation(Operation op)
+void KissCount::UpdateOperation(Operation& op)
 {
   _db->UpdateOperation(op);
 }
 
-wxString KissCount::AddOperation(Operation op)
+wxString KissCount::AddOperation(Operation& op)
 {
   return _db->AddOperation(_user, op);
 }
 
-void KissCount::DeleteOperation(Operation op)
+void KissCount::DeleteOperation(Operation& op)
 {
   _db->DeleteOperation(op);
 }
@@ -99,12 +99,12 @@ void KissCount::DeleteOperations(int month, int year)
     }
 }
 
-void KissCount::SetAccountAmount(int month, int year, wxString accountId, double amount)
+void KissCount::SetAccountAmount(int month, int year, const wxString& accountId, double amount)
 {
   _db->SetAccountAmount(month, year, accountId, amount);
 }
 
-wxString KissCount::AddAccount(Account ac)
+wxString KissCount::AddAccount(Account& ac)
 {
   ac.id = _db->AddAccount(_user, ac);
   _user->_accounts.push_back(ac);
@@ -112,7 +112,7 @@ wxString KissCount::AddAccount(Account ac)
   return ac.id;
 }
 
-void KissCount::UpdateAccount(Account ac)
+void KissCount::UpdateAccount(Account& ac)
 {
   std::vector<Account>::iterator it;
   int i;
@@ -123,7 +123,7 @@ void KissCount::UpdateAccount(Account ac)
       _user->_accounts[i] = ac;
 }
 
-void KissCount::DeleteAccount(Account ac)
+void KissCount::DeleteAccount(Account& ac)
 {
   std::vector<Account>::iterator it;
   int i;
@@ -137,7 +137,7 @@ void KissCount::DeleteAccount(Account ac)
       }
 }
 
-wxString KissCount::AddCategory(Category category)
+wxString KissCount::AddCategory(Category& category)
 {
   wxString id;
   id = _db->AddCategory(_user, category);
@@ -148,7 +148,7 @@ wxString KissCount::AddCategory(Category category)
   return id;
 }
 
-void KissCount::UpdateCategory(Category category)
+void KissCount::UpdateCategory(Category& category)
 {
   _db->UpdateCategory(category);
   
@@ -160,7 +160,7 @@ void KissCount::UpdateCategory(Category category)
       }
 }
 
-void KissCount::DeleteCategory(Category category)
+void KissCount::DeleteCategory(Category& category)
 {
   _db->DeleteCategory(_user, category);
 
@@ -206,23 +206,23 @@ void KissCount::GenerateMonth(int monthFrom, int yearFrom, int monthTo, int year
   _wxUI->GenerateMonth(monthTo, yearTo);
 }
 
-void KissCount::ChangePassword(wxString password)
+void KissCount::ChangePassword(const wxString& password)
 {
   _db->ChangePassword(_user, password);
 }
 
-bool KissCount::UserExists(wxString name)
+bool KissCount::UserExists(const wxString& name)
 {
   return _db->UserExists(name);
 }
 
-void KissCount::ChangeName(wxString name)
+void KissCount::ChangeName(const wxString& name)
 {
   _db->ChangeName(_user, name);
   _user->_name = name;
 }
 
-void KissCount::NewUser(wxString name)
+void KissCount::NewUser(const wxString& name)
 {
   wxDateTime curDate;
   Account ac = {wxT(""), wxT("Account 1"), wxT(""), false, true};
diff --git a/src/controller/KissCount.h b/src/controller/KissCount.h
index 6edd211..081260e 100644
--- a/src/controller/KissCount.h
+++ b/src/controller/KissCount.h
@@ -16,30 +16,30 @@ class KissCount
   ~KissCount();
 
   std::list<wxString> GetUsers();
-  bool IsValidUser(wxString user, wxString password);
-  void LoadUser(wxString user);
+  bool IsValidUser(const wxString& user, const wxString& password);
+  void LoadUser(const wxString& user);
   User* GetUser();
-  void ChangePassword(wxString password);
-  bool UserExists(wxString name);
-  void ChangeName(wxString name);
-  void NewUser(wxString name);
+  void ChangePassword(const wxString& password);
+  bool UserExists(const wxString& name);
+  void ChangeName(const wxString& name);
+  void NewUser(const wxString& name);
 
   void LoadYear(int year, bool force=false);
 
-  wxString AddOperation(Operation op);
-  void UpdateOperation(Operation op);
-  void DeleteOperation(Operation op);
+  wxString AddOperation(Operation& op);
+  void UpdateOperation(Operation& op);
+  void DeleteOperation(Operation& op);
   void DeleteOperations(int month, int year);
 
-  double GetAccountAmount(wxString id, int month, int year);
-  void SetAccountAmount(int month, int year, wxString accountId, double value);
-  wxString AddAccount(Account ac);
-  void UpdateAccount(Account ac);
-  void DeleteAccount(Account ac);
+  double GetAccountAmount(const wxString& id, int month, int year);
+  void SetAccountAmount(int month, int year, const wxString& accountId, double value);
+  wxString AddAccount(Account& ac);
+  void UpdateAccount(Account& ac);
+  void DeleteAccount(Account& ac);
 
-  wxString AddCategory(Category category);
-  void UpdateCategory(Category category);
-  void DeleteCategory(Category category);
+  wxString AddCategory(Category& category);
+  void UpdateCategory(Category& category);
+  void DeleteCategory(Category& category);
 
   std::map<int, std::vector<int> > GetAllOperations();
 
diff --git a/src/model/Database.cpp b/src/model/Database.cpp
index b17c6b7..6e9d691 100644
--- a/src/model/Database.cpp
+++ b/src/model/Database.cpp
@@ -113,7 +113,7 @@ void Database::CreateDatabase()
 }
 
 
-wxString Database::HashPassword(wxString password)
+wxString Database::HashPassword(const wxString& password)
 {
   blk_SHA_CTX sha_ctx;
   unsigned char sha[20];
@@ -148,7 +148,7 @@ std::list<wxString> Database::GetUsers()
   return res;
 }
 
-bool Database::IsValidUser(wxString user, wxString password)
+bool Database::IsValidUser(const wxString& user, const wxString& password)
 {
   bool res;
   wxString req;
@@ -164,7 +164,7 @@ bool Database::IsValidUser(wxString user, wxString password)
   return res;
 }
 
-User* Database::LoadUser(wxString name)
+User* Database::LoadUser(const wxString& name)
 {
   wxSQLite3ResultSet set;
   wxString req;
@@ -299,7 +299,7 @@ void Database::LoadYear(User* user, int year)
   set.Finalize();
 }
 
-double Database::GetAccountAmount(wxString id, int month, int year)
+double Database::GetAccountAmount(const wxString& id, int month, int year)
 {
   wxSQLite3ResultSet set;
   wxString req;
@@ -325,7 +325,7 @@ double Database::GetAccountAmount(wxString id, int month, int year)
   return res;
 }
 
-void Database::UpdateOperation(Operation op)
+void Database::UpdateOperation(Operation& op)
 {
   wxString req;
   req = wxT("UPDATE operation SET ") ;
@@ -345,7 +345,7 @@ void Database::UpdateOperation(Operation op)
   EXECUTE_SQL_UPDATE(req, );
 }
 
-wxString Database::AddOperation(User* user, Operation op)
+wxString Database::AddOperation(User* user, Operation& op)
 {
   wxString req, res;
   wxSQLite3ResultSet set;
@@ -394,7 +394,7 @@ wxString Database::AddOperation(User* user, Operation op)
   return res;
 }
 
-void Database::DeleteOperation(Operation op)
+void Database::DeleteOperation(Operation& op)
 {
   wxString req;
   req = wxT("DELETE FROM operation WHERE id='") + op.id + wxT("'");
@@ -438,7 +438,7 @@ void Database::DeleteOperations(User* user, int month, int year)
 
 }
 
-void Database::SetAccountAmount(int month, int year, wxString accountId, double amount)
+void Database::SetAccountAmount(int month, int year, const wxString& accountId, double amount)
 {
   wxString req;
   req = wxT("UPDATE account_amount SET ") ;
@@ -468,7 +468,7 @@ void Database::SetAccountAmount(int month, int year, wxString accountId, double
     }
 }
 
-wxString Database::AddAccount(User* user, Account ac)
+wxString Database::AddAccount(User* user, Account& ac)
 {
   wxString req, res;
   wxSQLite3ResultSet set;
@@ -504,7 +504,7 @@ wxString Database::AddAccount(User* user, Account ac)
   return res;
 }
 
-void Database::UpdateAccount(Account ac)
+void Database::UpdateAccount(Account& ac)
 {
   wxString req;
   req = wxT("UPDATE account SET ") ;
@@ -523,7 +523,7 @@ void Database::UpdateAccount(Account ac)
   EXECUTE_SQL_UPDATE(req, );
 }
 
-void Database::DeleteAccount(Account ac)
+void Database::DeleteAccount(Account& ac)
 {
   wxString req;
   req = wxT("DELETE FROM account WHERE id='") + ac.id + wxT("'");
@@ -531,7 +531,7 @@ void Database::DeleteAccount(Account ac)
   EXECUTE_SQL_UPDATE(req, );
 }
 
-wxString Database::AddCategory(User* user, Category category)
+wxString Database::AddCategory(User* user, Category& category)
 {
   wxString req, res;
   wxSQLite3ResultSet set;
@@ -567,7 +567,7 @@ wxString Database::AddCategory(User* user, Category category)
   return res;
 }
 
-void Database::UpdateCategory(Category category)
+void Database::UpdateCategory(Category& category)
 {
   wxString req;
   wxString color;
@@ -587,7 +587,7 @@ void Database::UpdateCategory(Category category)
   EXECUTE_SQL_UPDATE(req, );
 }
 
-void Database::DeleteCategory(User* user, Category category)
+void Database::DeleteCategory(User* user, Category& category)
 {
   wxString req;
 
@@ -733,7 +733,7 @@ void Database::GenerateMonth(User* user, int monthFrom, int yearFrom, int monthT
     }
 }
 
-void Database::ChangePassword(User* user, wxString password)
+void Database::ChangePassword(User* user, const wxString& password)
 {
   wxString req;
 
@@ -744,7 +744,7 @@ void Database::ChangePassword(User* user, wxString password)
   EXECUTE_SQL_UPDATE(req, );
 }
 
-bool Database::UserExists(wxString name)
+bool Database::UserExists(const wxString& name)
 {
   wxSQLite3ResultSet set;
   wxString req;
@@ -764,7 +764,7 @@ bool Database::UserExists(wxString name)
   return res;
 }
 
-void Database::ChangeName(User* user, wxString name)
+void Database::ChangeName(User* user, const wxString& name)
 {
   wxString req;
 
@@ -775,7 +775,7 @@ void Database::ChangeName(User* user, wxString name)
   EXECUTE_SQL_UPDATE(req, );
 }
 
-void Database::NewUser(wxString name)
+void Database::NewUser(const wxString& name)
 {
   wxString req, id;
   wxSQLite3ResultSet set;
diff --git a/src/model/Database.h b/src/model/Database.h
index 5ca3ebc..6e5cbdf 100644
--- a/src/model/Database.h
+++ b/src/model/Database.h
@@ -19,40 +19,40 @@ class Database
   Database();
 
   std::list<wxString> GetUsers();
-  bool IsValidUser(wxString user, wxString password);
+  bool IsValidUser(const wxString& user, const wxString& password);
 
-  User* LoadUser(wxString name);
+  User* LoadUser(const wxString& name);
   void LoadYear(User* user, int year);
 
-  void UpdateOperation(Operation op);
-  wxString AddOperation(User* user, Operation op);
-  void DeleteOperation(Operation op);
+  void UpdateOperation(Operation& op);
+  wxString AddOperation(User* user, Operation& op);
+  void DeleteOperation(Operation& op);
   void DeleteOperations(User* user, int month, int year);
-  double GetAccountAmount(wxString id, int month, int year);
-  void SetAccountAmount(int month, int year, wxString accountId, double amount);
+  double GetAccountAmount(const wxString& id, int month, int year);
+  void SetAccountAmount(int month, int year, const wxString& accountId, double amount);
 
-  wxString AddAccount(User* user, Account ac);
-  void UpdateAccount(Account ac);
-  void DeleteAccount(Account ac);
+  wxString AddAccount(User* user, Account& ac);
+  void UpdateAccount(Account& ac);
+  void DeleteAccount(Account& ac);
 
-  wxString AddCategory(User* user, Category category);
-  void UpdateCategory(Category category);
-  void DeleteCategory(User* user, Category category);
+  wxString AddCategory(User* user, Category& category);
+  void UpdateCategory(Category& category);
+  void DeleteCategory(User* user, Category& category);
 
   std::map<int, std::vector<int> > GetAllOperations(User* user);
   void GenerateMonth(User* user, int monthFrom, int yearFrom, int monthTo, int yearTo);
 
-  void ChangePassword(User* user, wxString password);
-  bool UserExists(wxString name);
-  void ChangeName(User* user, wxString name);
-  void NewUser(wxString name);
+  void ChangePassword(User* user, const wxString& password);
+  bool UserExists(const wxString& name);
+  void ChangeName(User* user, const wxString& name);
+  void NewUser(const wxString& name);
 
   void KillMe(User* user);
  private:
   wxSQLite3Database _db;
     
   void CreateDatabase();
-  wxString HashPassword(wxString password);
+  wxString HashPassword(const wxString& password);
 };
 
 #endif
diff --git a/src/model/User.cpp b/src/model/User.cpp
index 1937981..4b3f7a8 100644
--- a/src/model/User.cpp
+++ b/src/model/User.cpp
@@ -13,7 +13,7 @@ User::~User()
     }
 }
 
-Category User::GetCategory(wxString catId)
+Category User::GetCategory(wxString& catId)
 {
   Category cat;
   std::vector<Category>::iterator it;
@@ -31,7 +31,7 @@ Category User::GetCategory(wxString catId)
   return cat;
 }
 
-wxString User::GetCategoryName(wxString catId)
+wxString User::GetCategoryName(wxString& catId)
 {
   std::vector<Category>::iterator it;
   for (it=_categories.begin(); it !=_categories.end(); it++)
@@ -41,7 +41,7 @@ wxString User::GetCategoryName(wxString catId)
   return _("Unknown") ;
 }
 
-wxString User::GetCategoryId(wxString catName)
+wxString User::GetCategoryId(wxString& catName)
 {
   std::vector<Category>::iterator it;
   for (it=_categories.begin(); it !=_categories.end(); it++)
@@ -51,7 +51,7 @@ wxString User::GetCategoryId(wxString catName)
   return wxT("0") ;
 }
 
-wxString User::GetAccountName(wxString accountId)
+wxString User::GetAccountName(wxString& accountId)
 {
   std::vector<Account>::iterator it;
   for (it=_accounts.begin(); it !=_accounts.end(); it++)
@@ -61,7 +61,7 @@ wxString User::GetAccountName(wxString accountId)
   return _("Unknown") ;
 }
 
-wxString User::GetAccountId(wxString accountName)
+wxString User::GetAccountId(wxString& accountName)
 {
   std::vector<Account>::iterator it;
   for (it=_accounts.begin(); it !=_accounts.end(); it++)
diff --git a/src/model/User.h b/src/model/User.h
index 26207e4..b3e3b05 100644
--- a/src/model/User.h
+++ b/src/model/User.h
@@ -23,11 +23,11 @@ class User
   std::vector<Category> _categories;
   std::map<wxString, wxString> _preferences;
 
-  Category GetCategory(wxString catId);
-  wxString GetCategoryName(wxString catId);
-  wxString GetCategoryId(wxString catName);
-  wxString GetAccountName(wxString accountId);
-  wxString GetAccountId(wxString accountName);
+  Category GetCategory(wxString& catId);
+  wxString GetCategoryName(wxString& catId);
+  wxString GetCategoryId(wxString& catName);
+  wxString GetAccountName(wxString& accountId);
+  wxString GetAccountId(wxString& accountName);
   int GetCategoriesNumber();
   int GetAccountsNumber();
   int GetOperationsNumber(int month, int year);