I can't believe all the solutions which have been mentioned so far as the "simplest".
The SIMPLEST solution, is to teach your driver to live with it, and account for the drift when he/she is on the field. It's not that difficult...
Human controlled, dynamically adjusted voltage scaling. (Also known as HuCDAVS), is a simple control method that involves holding one joystick a little bit further forward than the other.
John