It's too bad this has pretty much died...I know a lot of people used to enjoy it. It was a good excuse for me to get off my lazy (read=busy) rearend and do something creative.
Alpha masking might help you out. CMD has a tutorial on his site... Hope this helps!
http://cmdsketchpad.com/tutorial.php?tutid=2
http://cmdsketchpad.com/tutorial.php?tutid=3