Pause the game, hold [Shift] and type in:

 S66S     Fat Tires
 4awd     Perfect Balance
 4w62     Add 8 Minutes to Run
 648w     Unlock Burnside
 648a     Unlock School
 648d     Unlock Warehouse
 648s     Unlock Granny
 as86wa84 Always Full Special
 466wss   Score X 10
 ssw664   Score / 10

 Keys:

 W - Pedal
 S - Brake
 A - Left
 D - Right
 2 - Bunnyhop
 4 - Air Trick
 6 - Spin Trick
 8 - Grind/Stall