WP7: Pass parameter to new page?
In his book "Programming Windows Phone 7" (chapter 6, section 3, "Sharing Data Among Pages") Charles Petzold recommends properties in the App class (derived from Application). Every page has access to it via Application.Current. Also interesting is the dictionary PhoneApplicationService.Current.State. It's usefull for tombstoning. The whole chapter maybe interesting for reading.
You should also look at MVVM pattern and the messenger class.
Here's some references:
MVVM Overview
MVVM Foundation Messenger
Good SO question on the messenger from MVVM Light
MVVM Light Blog