Armor, Health and Money
The three most important things in the game are armor, health and money. Armor prevents your health from depleting, health keeps you alive and money lets you purchase weaponry and gain street credibility.
In the game, enter "R1, R2, L1, X, Left, Down, Right, Up, Left, Down, Right, Up" and you will instantly gain extra units of all three of these elements. Repeat as often as you like to continuously increase your money supply.
Wanted Levels
Wanted levels determine the amount of police officers that will be on the lookout for you and your vehicle. As you kill pedestrians and wreck cars, your wanted levels increase, creating a larger problem during play as police will be coming at you from all angles.
Press "Circle, Right, Circle, Right, Left, Square, Triangle, Up" to keep your wanted level low no matter how much damage you do. This is especially helpful on time trials and other parts of the game that require quick movement with little hassle from the police.
Perfect Handling
Perfect handling allows you to better control your vehicle. You usually have to unlock vehicles with better speed and handling as you play, but entering "Triangle, R1, R1, Left, R1, L1, R1, L1" will instantly give any vehicle perfect handling. This allows you to maneuver through city streets and alleys with ease, preventing accidents that can cause a police chase.
Faster Cars
The fastest cars are usually available only late in the game, but if you enter "Right, R1, Up, L2, L2, Left, R1, L1, R1, R1" your car will move faster, regardless of your level. This is especially helpful on time trials and when evading police officers.