Fast Start
To begin faster than everyone else, at the start of a race just as the "Go!" sign appears, hold the control stick forward and press the A button as fast as you can.

All Characters, Boards, And Tracks
Enter this cheat at the title screen where it says start:
Down On The Control Stick, Up On The Control Stick, Down On The +Pad, Up On The +Pad , Down C, Up C, L, R, Z, Left On The +Pad, Right C, Up On The Control Stick, B, Right On The +Pad, Left C, then press Start. If successful you will hear "Yeah".