How to implement trialware model on Android?

Any tips on how to implement a trialware model for Android apps? I plan to release my app as a free app that expires after 30 days unless the user buys the license via a开发者_如何转开发n in-app purchase.

I can use the Android Market API to tell if they bought the license, so my main question is how to prevent the user from un-installing and re-installing my app every 30 days? Can I save something to their phone in a permanent and reliable way that will remain on the phone even if they uninstall? I know nothing will stop a determined hacker, I just want to stop the average user.

I am also open to different approaches to going trialware on Android.

Thanks in advance,

Barry

Have you read the documentation on Application Licensing? Specifically, have a look at Implementing a Policy

I also have found this resource very helpful in determining the best way to get a unique ID for any one device. Android - Identifying App Installations -- They discuss pros/cons to the different approaches -- Straight from the developer's mouths!

I would avoid leaving unwanted remains of the application on their phone.

Instead, you can take some unique identifier of the device, and send it to your server, if this ID was not registered before, send it activation code, if it was, don't send it.

Store this activation code in some shared preferences etc. and when the application starts, check if the activation code is there and if it is valid.

They don't even have to uninstall, just clean app data. You can save something to an obscure location on the SD card, but that's also fairly easy to circumvent, even easier if they read this forum :) Your best bet is to have the app call home to your server and check if licensed periodically. That of course comes with it's own problems: do you allow it to run if network connection is never available, etc.