Scooters:
Press C-Down, C-Right, C-Up, C-Left, Z(2), L, C-Left at the menu screen.

Super fast bikes:
Press C-Up, C-Left(2), C-Right, L, R, C-Down, Z at the menu screen. 

All bikes and tracks:
Press C-Up, C-Left(2), C-Right, L, R, C-Down, Z at the menu screen. 

Insanity level:
Press C-Up, C-Left, C-Left, C-Right, L, R, C-Down, Z at thrash mode.

Be The Cop mode:
Press Z, C-Left, C-Down, C-Left, Z, L, R, C-Down at the menu screen. 

Girl biker:
Press C-Right, C-Left, Z, L, R, C-Up at the menu screen. 

Level 2:
Press R, C-Right, Z, R, L, C-Up, C-Left, C-Up at the menu screen. 

Level 3:
Press R, C-Right(3), R, C-Left, C-Down, Z at the menu screen. 

Level 4:
Press R, C-Right, C-Down, C-Left, C-Right(2), Z, L at the menu screen. 

Level 5:
Press Z, C-Right, C-Down, C-Left, C-Right(2), Z, L at the menu screen. 

Quick start:
Hold A at the beginning of a race to get a faster start.

Alternate bike and racer colors:
Press Up or Down at the bike selection screen in any mode.

Stay on the ground:
Hold L after falling off your bike to remain on the ground.

Avoid sharp turn crashes:
Press A after most sharp turns to balance out your bike.

Avoid car crashes:
Hold A just before hitting a car to perform a wheelie and use the car as a 
ramp jump. Note: This maneuver will not work on log trucks or big rigs.

Safe landing:
If you get knocked high in the air and are spinning wildly, repeatedly tap 
B to straighten out for a better landing.

Stay on your bike:
When you are about to crash rapidly and repeatedly press A + B to have a 
better chance of landing on your wheels.

After getting launched high in the air, you have a better chance of 
staying on your bike by reducing your speed while in the air to zero and 
releasing the brake once your bike hits the ground. 

More height on jumps:
When you jump over cars or other objects, you will get more height if you 
wheelie and release the throttle before you jump. 

Big bikes:
Successfully complete all the races on big game mode to unlock two big 
bikes that can go over 300 mph.

Shortcut and 4X power-up:
Begin racing on level five, race three. Note the sharp turn sign in front of 
a red and blue striped building, about halfway through the city. Right after 
that point there is a hole in a building with dark green around it on the 
right side of the screen. Go through it for a shortcut. There is a 4X 
power-up inside the shortcut. Be careful, as there are goal posts from a 
football stadium right when you go through the hole. Note: This may not be 
done in multi-player mode.

Beat Down level:
Ride through the grass in the Beat Down level. The police will ignore your 
biker and stop everyone else. 

Disable cop:
Pop a wheelie when you see a cop waiting on the shoulder to knock him out. 

Disable opponents:
Once you receive the taser, mace, or cattle prod, then you can employ the 
following strategy. Once the race begins, use either the mace, taser, or 
cattle prod on the other bikers. Once you do that, quickly press C-Right 
to punch them, but you will still have the weapon selected. This will cause 
them to veer off the road and perhaps hit a car or other obstacle. This is 
very effective when there are cops around, since this will allow them to 
easily bust the other bikers while they are disabled. 

Rap Sheet entries:

Ride next to a car and hit it with a weapon for a Cage Rattler entry.
Run over a pedestrian for a Hit and Run entry.
Get a weapon stolen for Butterfingers entry.
Knock a cop off his bike for a Most Wanted entry.
Hit a cop to have Assaulted Officer entry.
Hit someone to have a Battery entry
Wreck someone to have an Accident entry.
Call off a cop to have a Resisted Arrest entry.
Steal a weapon to have a Master Thief entry. 

Make cows moo:
Go to a level where there are cows. When you see them, jump off of one to 
make it moo.

Steal weapons:
Get your weapon to your hand during game play, then hold C-Right until someone 
tries to hit you with their weapon. If you are lucky, you will steal their 
weapon and get to use it on opponents. You will also get a "Master Thief" 
rating on your rap sheet if you steal a weapon. 

Multi-wheelies:
Select a three or four player thrash game. When the starting timer reaches two, 
have all players hold Z + A simultaneously. If done correctly, all the bikes 
will pop a wheelie and freeze the game.