Tyler's plan is solid for building a good strength base and getting a feel for the false grip. But the best aid I've found in teaching the MU is actually the clapping pullup (http://www.crossfit.com/mt-archive2/clap.html
By the time most people begin working the MU, they can get off at least a couple of dips with sufficient depth to conquer the push portion of the movement. The problem is usually in the pull, where they don't generate enough height and momentum to make it through the transition.
Clapping pullups, then, give you a great way to practice getting higher in your pulling motion; they force you to generate power in a way that regular pullups don't.
My only other tip: consider trying some jumping muscle-ups, using decreasing amounts of jump. There's a definite skill component to the transition portion of the MU, and being able to practice it repeatedly makes the move to non-jumped (i.e. 'real') MU's far easier.