What are your experiences/thoughts ?
Are there currently any approaches that prevent automated cracks ? I know that any app is crackable if someone is persistent. However, i would like to make it as hard as possible for the crackers.
What methods are currently used for integrity checking and crack prevention ?
Basically it's impossible to completely stop crackers from cracking your app if they really want it, but there are many ways to slow them down, especially so that it doesn't get cracked within the first few hours of release on the AppStore, which happens a lot.
Here's a general page to show some basics: http://iphonedevwiki.net/index.php/Crack_prevention
Given that users have physical access to a device running your app, crack prevention may well be impossible for any app that doesn't require valuable external resources outside of the users control.
To help reduce the likelihood of automated cracks, don't use any published methods exactly as is; but change them all slightly to prevent their signature from being detected. Spread many different methods that don't share any common code throughout the app, and have some of them cause delayed (days, weeks) and difficult to recognize changes in how the app behaves.
Note that any effort in time (beyond some small minimum) to add and debug such code may not bring in any compensating revenue and may potentially make your app less stable for all users (including those who write reviews).