Enter "911" as a number. An ambulance will arrive to heal you for a fee.
Complete all Top Model driving missions.
Complete all Vehicle Transport driving missions.
Discover all car dealerships.
Push start then press B, X, Y, RT, LT, X, A then push resume play and all cars will be unlocked.
Win the Tour of the Island race.
Go to the Equipment Modifiers screen. Once there, you are able to take the equipment bonuses off of equipment that you don't have, but have unlocked already, and put them onto equipment you do have. You can use this to get 10% bonuses to all stats early on.
Go to the password screen and enter ITSINTHEGAME