You will need all of your save slots to do this. At the copyright screen, press Left, Left, Right, R, R to clear the saves. Name the saves these, in order, 1st Slot: BEST 2nd Slot: GAME 3rd Slot: EVER .
You can now delete the second and third saves. Name these saves as follows, depending on what cheat you want.
If you want infinite ammo and infinite life, name one of the saves SUPERSPY. If you want arcade level, name one of the saves EDACRA . If you want all alternate vehicles, name one of the saves G6155 .