Besides the technical aspects of implementing App Store subscriptions, there's quite a bit of business logic you have to understand. Daniel does a good job of pointing out how important it is to be clear about billing and trials. Also, it's important to get the wording right when presenting the subscription payment screen.
I have personally consulted on an app that ran into big trouble with the way they implemented over-priced subscriptions. Because of another issue with the app, they weren't able to update the app at all without big changes, leaving tons of money on the table. You need to know this stuff before you implement or risk being pulled or maybe even banned from the App Store.
App Store Subscriptions And You by Daniel Kennett