First of all look at the arena size in which you'll play the matches. Depending upon that you need to figure out what speed does your bot requires. For Example if your arena is too small then keep the speed low/moderate(as per your choice) and vice versa..this well help maneuverability and cuts down the chances of you getting knocked off the arena. Further than this you must keep it simple,get your drive-train right and have enough torque to push the other bots out...possibly a wedge(ramp) at the front side of your robot would do. Avoid loose mounting of motors and other parts.
Well that would be it.
Best Luck and have a great time!!