I would suggest ensuring that you have feedback on each of the wheels. On our 3 wheel omni platform we drove it with and without velocity feedback (RobotC has this built in) and the difference was night and day.
http://www.youtube.com/watch?v=qobHqf45SLc is before we put feedback on... but here is the one with feedback.
http://www.youtube.com/watch?v=Qwg2cgNTAZE
In both cases, disregard the audio tracks they don't really add anything...