[ChrisH]

In addition to the previous suggestions, you might try putting a slight "crown" on your rollers. Particularly the ones at the ends of the robot. It doesn't take much. If the center of the rollers is slightly larger in diameter than the sides, then the belt tends to self center. Most conveyour belts do this but the crown is so slight you can barely see it. 1-5% of the diameter would probably do it.