Technology to make my Podcast Protected

We have a real life on-the-air Radio program for whcih we provide podcasts to listeners. We'd like to come up with a mechanism wherby the users will need to pay for the podcasts. Additionally it would be nice if we could have some way of giving out "coupons" that will allow some listeners to get that episode for free. It would also be nice if there were a way to automatically let them listen to the first 15 minutes for FREE and then charge them to download the remainder. The content should ideally be protected so that it cannot be redistributed/pirated by one listner who downloads and gives copies of it to his friends. We are open to other ideas/suggestions on how to implement this?