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Here is a short tip on how to detect if a tombstone happened.

Source: Windows Phone Developer FAQ

You mentioned not to use StartupMode to detect a tombstone, how should it be done?

If you just want to detect if a tombstone happened, simply set a flag in your App’s handler to Deactivated event. If the value of the flag has not been reset (in Activated event) , then the app was not terminated.

Jeremy Likness continues his ultra light Windows Phone 7 MVVM Framework article with this post on tombstoning with MVVM and Sterling.

Source: C#er : IMage

Sterling makes tombstoning very easy because it handles serialization of just about any type of object. To show an example, we'll start with the concept of a view model that holds a set of categories (that a pivot is bound to) and a set of items that are filtered by category. When tombstoned, the application must remember the category as well as any item that is selected.

In the fourth and final article of Jeff Prosise's series on tombstoning, he fixes one last bug and finally produces a finished application.

Source: Jeff Prosise's Blog

Tombstoning is one of the greatest challenges in writing applications for Windows phones, which is why I decided to devote a series of blog posts to it. In Part 1 of this series, we built a photo-extras application that allows the user to perform simple image-editing chores on photos. In Part 2, we added tombstoning support so the application wouldn’t lose its state when tombstoned. And in Part 3, we fixed a bug in our tombstoning logic and refined it so that it wouldn’t untombstone anything if the app hadn’t been tombstoned in the first place.

Jeff Prosise has published the third part of his series on tombstoning in Silverlight for Windows Phone.

Source: Jeff Prosise's Blog

In Part 2 of this series, we made progress toward including tombstoning support in a photo-extras application by adding OnNavigatedFrom and OnNavigatedTo methods that use a combination of page state and isolated storage to save the state of our app before it’s deactivated and restore that state following reactivation. But we also determined that work remains to be done. Specifically, our tombstoning code prevented the user from opening another picture.

This article is compatible with the latest version of Silverlight for Windows Phone 7.

This article is Part 3 of “A classic memory game”:

• A classic memory game: Part 1 – Designing the game
• A classic memory game: Part 2 – The game logic, connecting the ViewModel and the View
• A classic memory game: Part 3 – Porting the game to Windows Phone 7

The series is about building the following classic memory game in Silverlight, and porting it to Windows Phone 7. In the first article we started a new MVVM Light project, created the controls and designed the game-states. In the second part we put the logic behind the game, and connected it with the View thus completing the Silverlight version of the game.

Jeff Prosise has two blog posts discussing tombstoning in Silverlight for WP7:

• Tombstoning Pivot Controls in Silverlight for Windows Phone 
• Tombstoning Panorama Controls in Silverlight for Windows Phone 

  What is tombstoning? In short, while Windows phones are perfectly capable of running several applications at once, only OEMs can write apps that run in the background. Apps written by ordinary mortals can only run one at a time. If the user switches away from your app by, say, pressing the phone’s Start button, your app is terminated. To be more precise, the app is deactivated, and if the user next presses the Back button, the app is reactivated. But because deactivation really does mean the app was terminated, all state – including the state of the app’s controls – is lost. We refer to the deactivation-reactivation cycle as tombstoning.

   

Joost van Schaik has made some universal way of tombstoning for MVVM Light.

Source: .NET by Example

I’ve been down this road before but I thought it wise to revisit this subject, since I see a lot of people in the Windows Phone 7 developer community still struggling with the concept of tombstoming. In my previous attempt to make some universal way of tombstoning ViewModels on Windows Phone 7 I used DataContractSerializer, which is a nice idea but does not work very well with MVVMLight since the ViewModelBase is not serializable. And sometimes you have to go trough a lot of hooplah if stuff you use in your code does not turn out to be serializable after all.There is a better way, I think. So here it is: universal tombstoning for MVVMLight, take two.

In this post, Karl Erickson goes into detail in the areas of Windows Phone 7 navigation structure, tombstoning, and data loading, and provides some tips to help you avoid common problems.

Source: MSDN Blogs

When you start learning a new platform, the documentation can only take you so far. The Windows Phone 7 docs are great, but when you start developing your first application, you will inevitably run into issues that the documentation doesn't cover. We are creating the Fuel Tracker guidance application to help people through this process. The idea is to address some of the most common issues that come up for all applications.

Peter Kuhn demonstrates an attribute-driven approach to mark members of types for persistence in the transient application state of WP7, similar to using the data member attribute when the data contract serializer is used.

Source: Mister Goodcat

Hopefully every Windows Phone 7 developer has heard about Tombstoning by now. To avoid writing the same code to persist and retrieve data again and again, there have been quite some efforts in the past to create generic methods of handling this process of application deactivation/termination by various people. James Ashley for example uses the ApplicationSettings and State classes for this (improved by rschiefer here). I personally like implementations that use the data contract serializer (here is an example by Joost van Schaik), because I can selectively choose what values should be serialized by applying the data member attribute only to those properties I want to be persisted.