Passing data from page to page

PhoneApplicationService.Current.State["yourparam"] = param
NavigationService.Navigate(new Uri("/view/Page.xaml", UriKind.Relative));

then in other page simply

var k = PhoneApplicationService.Current.State["yourparam"];

Personally I'd store the values entered on Page B in a model(object) that is also accessible to Page A.

Depending on how you're navigating to Page A the second time, one or more of the following may be usful to help understand passing values between pages:

How to pass the image value in one xaml page to another xaml page in windows phone 7?

Passing a complex object to a page while navigating in a WP7 Silverlight application

How to pass an object from a xaml page to another?

How to pass a value between Silverlight pages for WP7?

How do I navigate from one xaml page to another, and pass values?

One thing you can consider is to use MVC: let your App be the controller, store all data in the model, and the pages are just views that contains pure UI logic. In this case your pages are painters and you pass your model object around. This gives nice isolation of business logic and the UI so that you can rev them easily.

BTW, Silverlight and XAML are great tools for MVC so it's a natural match.