Tasselfoot describes how to get 6k feet, and provides a video.
Strategy: After fully upgrading everything, let yourself go off the ramp normally at 6% resistance, glide for a few seconds to build height, then drop to 4 or 3% resistance. From there, use your rockets in small bursts to stay in the 130-150 speed range. You'll keep dropping though, but you should still be over 110 by the time you run out of fuel. Continue coasting, then when you start losing height, drop resistance to 0% to build up speed and get a good bounce off the water. Bounce, wait a second, then drop resistance back to 0% again. You should get 6000 before hitting the water a 2nd time, or if not... you should bounce a 2nd time and get to 6000 before hitting a 3rd time.Getting 6,000 Feet:
EASIER STRATEGY: Keep your wind resistance at 3% the entire time. Keep your speed between 175MPH and 200MPH. Doing this strategy should easily get you between 6,400 and 7,000 feet.