Just as the wheels touch the bottom of the jump, just as quick as you can flick the brake and get back on the gas. Not enough to slow down, just as fast as you can - brake-go.
This shifts weight forward and forces the nose down at the bottom of the jump so that as you go up the ramp and off, the nose stays up. Sometimes if you don't, when the springs compress on the bottom of the jump, they spring back and reach an "arc" about the middle of the ramp, lifting the nose. About this time the rear wheels have compressed against the bottom of the jump and are reacting by lifting the rear. This forces the nose back DOWN, so when you launch, the rear end rockets to the sky and it wants to roll over and nose in on the landing.
If you get airborne successfully - let off the gas like B.R.28 said (or you could blow an engine, or on landing strip a spur.) If it starts to nose up too fast and high, tap the brakes in mid-air. This works against the momentum of the spinning wheels to help level you out.
If it's still trying to roll with the nose down - give it a blip or two on the throttle while it's in the air, this can help bring the nose up.
If you're too far over either way nothing will help, might as well start walking now.
