I think the "words with friends" model works very well. There's a free version, it's identical except that it shows a small banner add on the main menu, and sho

I think the "words with friends" model works very well. There's a free version, it's identical except that it shows a small banner add on the main menu, and shows you a full-screen ad after every move. It's not very intrusive but you can't really ignore it either. It was enough to make me upgrade to the full ($2.99) version after playing for a week. And if users don't upgrade, you still get the ad revenue. Take a look if you haven't tried it yet.