Skate Or Die 2: The Search for Double Trouble Cheats

To do this you need either the Afterburner and/or Hangtime board.

Equip the Afterburner board and accelerate to maximum speed. Then switch to the Rocket or Hangtime board and you'll notice that you can infinitely keep increasing your speed.

This can also be done by starting with the Rocket board and then switching to the Hangtime board.