The past two seasons, team 148 has utilized an articulating drivetrain. The robot shifts from one set of wheels to another via pneumatics. One of these sets of wheels consists of (4) high-traction wheels, spread to the outer 4 corners of the robot in "narrow/long body" configuration. This configuration of wheels makes it VERY difficult for the robot to turn due to the scrub forces of these wheels -- in addition... it makes it hard for the robot to BE turned. This "traction mode" keeps the robot driving straight as an arrow.
If we are ever in a situation where we need to push forward in a straight line (like autonomous this year), we run in this traction mode. For normal moving around the field, we use our other drivetrain mode.
For more information on drivetrain turning, and scrub forces... check out the math in Chris Hibner's whitepaper:
http://www.chiefdelphi.com/media/papers/1443
Note... articulating drivetrains aren't for everyone. There is some added complexity over a "standard" style drive. However, we on 148 like the benefits that come with such a drive.
Whatever solution you decide to go with for driving straight, be it a software or mechanical solution, I strongly urge you to work to develop / test it during the off-season. What else were you going to do this summer anyways? Sleep?
-John