Maximize torque and weight within allowed limits, speed isn't quite as important, as long as you can catch up with your opponent.
Keep a good grip (i.e. "sticky" tires) and stay low compared to wheel base with your design, then you'll be harder to move/tilt.
Oh, and make sure you're absolutely scraping the floor - leave a gap for a wedge and someone will tilt you.
(You might wanna make the edges wedge shaped to tip opponents yourself).