Jump as high as you can to earn more points. In the air you can do spins to earn more points.
To jump, press the space bar when you start descending by the ramp, and keep it pressed until you're going up; you must release it just before reaching the end of the ramp to obtain a higher jump. If you press or release out of time, it will not be a great jump and you'll earn few points. When you are in the aire, use the arrow keys to do spins and tricks to earn extra points.