The best testability for your app you can achieve by separating your UI and business logic. One of the ways is to follow MVVM (Model-View-ViewModel) pattern, where UI (View) is separated from UI logic (ViewModel) and business layer (Model) so ViewModel and
Model are easily testable.
If you are not familiar with this pattern you can start from reading
Using the MVVM Pattern in Windows 8 article.
http://lunarfrog.com/blog/