Holonomic + gyro/PID

[biojae]

Originally Posted by AustinSchuh

We've seen wheelspin when accelerating quickly, and the wheels will slip when the robot gets hit hard. Both of those are relatively common in a match, so the encoders will no longer be at the correct position. This doesn't mean that encoders are useless sensors, but that they aren't a magic bullet. A clever algorithm would decide which one is more likely to be right, and use it to correct the other one based on the measured conditions.

Or, you could use follower wheels with encoders on them.
http://www.chiefdelphi.com/media/photos/32440