PROVIDING A TRIAL MODE
Including a trial mode for your app is a great way to give users a chance to try out your application before purchasing it. When you submit your application, you can tell Microsoft whether your application supports a trial mode. If you choose No, your application must be bought outright or not downloaded at all. If you choose Yes, customers can download your application for free, and you must selectively cripple your application such that users can see how good your application is, but without all the full-version features. You can provide the crippled trial version to entice customers to eventually purchase your application.
NOTE The simple method call bool isTrial = new LicenseInformation() .IsTrial(); tells you whether an application is in trial mode. Note that Microsoft recommends that you cache the results of this method call, as it can take a long time to compute.
Many people think that building a time-based trial application is a good way to go, but a user can simply uninstall and then reinstall the app to reset the timer and continue using the trial version. It's better to reduce the features in the trial version. For a game, you might not let users play all the levels or use all the available characters or weapons. (But remember that you want to show them what they're missing to get them hungrier for the full version.) For a productivity application, ...