That picture should make it fairly obvious. If not, look below. mainJoy is the one used for forward/back and strafing, rotJoy is used for rotation (pretty much like a FPS game, we even wired our camera's tilt up to rotJoy's Y axis so it is exactly like the usual FPS controls
) For best results, use an Xbox/PS3 controller. Remember forward is -1.
Assuming you get -1..1:
front_Left = -mainJoy.getY() - rotJoy.getX() + mainJoy.getX()
front_Right = -mainJoy.getY() + rotJoy.getX() - mainJoy.getX()
back_Left = -mainJoy.getY() - rotJoy.getX() - mainJoy.getX()
back_Right = -mainJoy.getY() + rotJoy.getX() + mainJoy.getX()