When you enter the board park, go all the way until you reach the ramps with the cars and trucks all around. After you pass one of the ramps, turn around and jump back up the hill. Repeat this until you hit the backside of the ramp. If you do it correctly you can get some pretty high air! It might take a while to do, but if you practice a lot you can even hit the roof of the game and you can do grabs and flips in the air.