Level select:
Start a new game and enter !EVAH! as a case-sensitive name.

Invincibility:
Start a new game and enter B4K42 as a case-sensitive name. The "God mode" selection will now be unlocked at the options screen when the game is paused.

All bonuses:
Start a new game and enter 105770Y2 as a case-sensitive name. Note: The code contains the number "zero" and not a capital letter "o". All character biographies, vehicle specifications, FMV sequences, and concept artwork will be unlocked.

Multiple codes:
Start a new game and enter more than one code together to activate each of them simultaneously. For example, enter B4K42105770Y2 as a case-sensitive name.

Hint: Jet mode:
The weapons in Jet mode are considerably less effective that the weapons in Walker mode. Couple this with the fact that Jet mode does not allow the Shield to generate. Only use Jet mode when you need to blow something up or kill a Fighter (or any other air units such as bombers, landers, etc.) in the air. Stay on your side of the battle, as it will be nice to have a helping hand when things get rough. Try not to use all your energy while in Jet mode; when you land you will have no shield. Repair Pads are your friends -- use them.