Speed! Go parralell to the pipe for about 5ft to start to pick up some speed then drop in, but dont turn 90 degrees and drop or youll flat bottom, instead drop at maybe a 10-20 degree angle, almost parralell to the pipe. When you drop make sure to try and contact the vert part of the pipe. Right after you drop in start pumping and carving towards the other wall. Play around with the angles, you dont want to go straight up in the air and you dont want to go flying down the pipe. That should get you some height if you understand what I just typed. The more speed you carry in to the pipe and the better you learn to pump the more height you will get.
'Computer games don't affect kids; I mean if Pac-Man affected us as kids, we'd all be running around in darkened rooms, munching magic pills and listening to repetitive electronic music.' -Kristian Wilson, Nintendo, Inc, 1989