Spherically-Sliced Athlete  -  You need to have registered a total score of 10000 first. Then while the event is loading, press one of the following sequences to replace your character with the spherically-sliced athlete: 

Up,Up,B,Down,Down,B - Yellow Up,Up,Up,B,Down,Down,Down,B -

Grey Up,Up,Up,Up,B,Down,Down,Down,Down - Pink

Speed Skiing Tricks
Speed up: 
You could increase the speed even further by bashing the d-button as well. 

Forward roll:-  During the announcement of your name, tap up-down-left-right-B. 

Side roll:-  Same as above, except tap the code backwards.

Ski Jump Tricks
Backwards somersaults:-  Whilst you are setting the angle of jump (ie. whilst pressing B), tap down-up, then jump off at 60 degrees. 

Increase distance:-  Hack the run buttons whilst in flight to increase distance. 

Mogule Jump:-  Same as 'backwards somersaults', except tap left-right. 

Super V Gliding:-  Tap right-left BEFORE you set the angle of jump.

Running Mode  -  During each event's title screen (ie. when the world record is shown), tap left-left-left-right-right-right.