All Other Cheats
Just tap all of the buttons repeatedly (R,L,Z,Cu,Cr,Cd,Cl)until the cheat is activated.
It works on all exept "Stunts". You may have to try it 5-10 times for it to work.

Obtain Keys Easier
There is an easy way to obtain the hard to get keys. You'll have to go to the cheat menu and select Resurrect in Place. Then select any track and locate a key in the air. Get directly underneath the key and press C-up many times (which will elevate you higher everytime you press C-up). You know have the key you couldnt reach.

Drive on Invisible Track
While driving on the stunt track, hit any of the ramps, preferably the purple half-pipe at a high rate of speed. Your car must be heading towards a wall. If you get enough height, you will easily clear the wall and fly into a black oblivion. You will be able to see the normal track if you look to the upper part of the screen. It is an awesome glitch that the devolpers should've picked up on.

Cheat Menu
To access the cheat menu, go to the setup screen and hold down L, R, and Z. While still holding them press all four of the C buttons. If done correctly under audio should appear cheats. To access the codes in the cheat menu simply put the cursor on the code you want to access and hold down L + R + Z. While holding those down press all four C buttons. Each code needs the C buttons pressed a different number of times. Below is the list:

Cheat 4 C buttons
Fog Color 3 times
Tire Scaling 6 times
Frame Scale 2 times
Game timer 3 times
Levitation 4 times
Resurrect in Place 2 times
Suicide mode 4 times
Mass 4 times
Super Tires 6 times
Invisible Tracks 7 times
Invisible car 8 times
Super speed 3 times
Auto abort 4 times
Track orientation 7 times
Gravity 5 times

There are also other cheats to be typed in. For each of the following you must have the corresponding cheat selected in the menu. Massive Mass - Hold L and R and press CU, CD, CL, CR
Killer Rats - Hold L and R and press Z, Z, Z, Z
New York Cabs - Press R, L, Z, CU, CD, CU

Secret Cars
Taxi - Get 4 keys in any level
Hot Rod - Get 8 keys in any level
Formula 1 - Get 10 keys in any level
Prototype - Get 12 keys in any level
Mt. Dew Dragster - Get all 4 Mt. Dew bottles in any level
Rocket Car - Get first in circuit mode