At the beginning of the level, get all the ammunition in the weapons room (to your left at the very start). Then, move on to the Major. When he tells you to go to the bunkers, take Ragman and the Doc and crouch down next to the Major. Then, take Junior and put him in the first bunker to your right and up the stairs. Then, take Hoss and put him in the next bunker down. Whenever someone dies, the Doc will heal them.
In the main menu, dont go anywhere, just put in L1 R1 L1 R1 square triangle circle triangle square circle, you should hear a couple drum beats, but do it kinda fast or it wont. Start or load a game, it can be used on either, pause the game, go to options. If you did it right, at the bottom it should say cheats. Open them up and find you can have unlimited ammo, saves, and choose from any mission, and even skip to the next one. I may be thinking of something else but I think you can see all the preview movies for each mission.