You will find plenty of discussion on Chief Delphi related to making your robot drive straight. This thread has been active lately.
http://www.chiefdelphi.com/forums/sh...t=cim+straight
To get a robot that always tracks perfectly straight is one of the fine arts of robotics. Typically you need some form of feedback, either encoders on the drive shafts or perhaps a gyro.
In my experience, though, if your robot isn't tracking reasonably straight it is as likely to be a mechanical problem of binding or unequal friction in the drive train as much as anything. In any case, you will see many other helpful suggestions if you search a bit. There may also be assistance on the FIRST Curriculum pages and the CD White Papers.
Jason