How to clear controls and value when navigating between screen.

Hi all,

I have developed a small application for practice which has login screen and once logged there will be sign out button.following are my questions

• When i click sign out now i simple navigated to the home login page using APP.to  but how to make sure when i click sign out i clear the variable are screens and return to home login page.
• When try to navigate to already existing page i get error saying cannot create duplicate controls and does not navigate to next screen.I am using XML views
• What the performance related to tips to be take care while create controls and navigating across views

Thanks

Deepan