For this you need 1 Gamecube controller. In a 2 Player match race, after using one controller to start the race, hold down the z button and you will be 2nd player until you let go of it. Then you will be first player again.
Get a green turtle shell from a mystery box. To drop it, hold Down and press A.
When you are going around a turn especially when you're on a dirty track, just hold the direction you are turing and rapidly tap the gas button. This helps tremendously when you're worried about skidding.
Though you have good turning already in this game, there is a way to get great turns and even cut off some time. When you come to a turn that may require some quick action, let go of the gas button (B) and do the turn. When you are in the turn or done it, hit the gas. This helps greatly to make the tough turns, and you can even get ahead of other players by doing this.
In Rainbow Road, do a match race. Just before you finish the final lap, fall off. The camera will go over the finish line saying you won. Lakitu will wave his checkered flag as your character floats back on to the track. Lakitu will not be holding him/her when he/she comes back on to the track.
On the character select screen go to the character you want to shrink and press 'Y' and 'A', You play as shrunken version of the character.
To reset your high scores, press A, B, L and R all at the same time on the game's title screen. If done correctly, your high scores will reset to the original setting.