Unlock all mechs & missions in Free Mission mode:
At the Title screen, switch the tuner to the following numbers and hold for one second each:

1-9-7-9-0-9-0-6

Position '0' is on the left end of the dial (9 o' clock position), position '4' is at the top of the dial (12 o' clock position) etc.

Movie viewer:
Beat the game on Normal difficulty. You'll find the Movie Viewer at the Configuration menu.

Voice select option:
Beat the game on Veteran difficulty. You'll find the Voice Select at the Configuration menu.

Desperado difficulty setting:
Beat the game on Veteran difficulty.

Lower mech:
Beat the game on Desperado difficulty. Press Hatch + Start during play to lower your mech.

Remove HUD:
Get rank of Lieutenant Colonel. Then, press Ignition + Start during play.

Juggernaut mech:
Get to rank to Colonel.

Rapier mech:
Get ten decorations.

Thanks to Revolution readers Thomas Hagan and Dire Wolf!