Little more information could help. . . .
Are you using the CIM's or Fisher Price as drive motors??
2, 4, or 6 wheel design??
What kind of Wheels/Thread are you using??
2 wheels powered and 2 casters??
Most teams do what is called Tank Steering.
What it normally is that when you want to turn left you reverse the left motor slightly and give forward power to the right motor.
It if the opposite for if you want to turn right.
This works great if you don't have too much friction.
Some threads you may want to look at:
http://www.chiefdelphi.com/forums/sh...ad.php?t=26096
http://www.chiefdelphi.com/forums/sh...ad.php?t=27636