If you're going to build a new (as in new to yourself) drive system, you must first know how it works. A good whitepaper that discusses the kinematics of a mecanum can be found here:
http://www.chiefdelphi.com/media/papers/1836.
Read it, learn it, know it. It will will help in your planning. Also, do a quick CD Media search for pictures of mecanums to get your brainstorming going.
Oh, not to mention the break-through 6wd-mecanum suspension, but
here's a good example. And then there's
488's prototype mecanum.