I am in favor of keeping the mechanics as simple as possible for someone learning. Here's what helped my daughter go from a frying pan serve with no pronation to a respectable serve.
The first thing was to simplify and go to a non-loop takeback. Basically a Roddick like motion where you take the racquet straight up rather than a big loopy backswing. The girl in this "serve doctor" video has nice simple takeback also. http://www.youtube.com/watch?v=Ixx-MCC7D88
The next big help was to get her to hit a slice serve. Hitting a flat serve she was always "palm up" at the top which kills your ability to pronate. Learning a slice and having to carve around the right side of the ball forced her to be in the proper position at the top.
It only took a couple of months and she developed a very reliable slice serve. From there it was relatively easy for her to learn to pronate and flatten it out. She has the proper motion now and is working on developing more power (leg bend, push up, shoulder rotation, etc.).