Tyler's muscle up program is good, and should have you doing MUs in short order.
The things that helped me most, however, were working on the false grip and slow negatives from support.
The false grip is key to a musle up. If you can't hold a false grip, you'll never be able to get "on top" of the rings in the transition from the pull up to the dip.
The slow negatives from support built my strength in the exact groove of he MU. Be sure to lower yourself down VERY slowly with the rings as close to you as possible. Also try to maintain the false grip all the way to the hang positoin when you do negatives.
You'll be doing musle ups in no time.