[Biff]

Ok, there is a lot of help in these threads, answers below are not all the ways you can do it.
1. If you can re do drive so moters turn the same way.
2.If you can't do that, use the dash board program, monitor under driver control to get values for speed you want and straight at the same time.
3. Add a gyro chip (search the treaeds for yaw rate or gyro)
4. Add Wheell or shaft encoders use them and negitive feed back
The last two require programing and working on the bot at a competition
#2 would be the fastest to emplement, but the least accruate.
We had a tank drive last year that would hardly trun on carpet, and with the drill moters driving it as you describe it would not go straight without cutting power to the motor turning clockwise as viewed from the back of the motor.
The drill motors are set up to drive screws in, so much more power and speed in the "tighten" direction.
Good luck