Gamer skill level:
Highlight the "Game Level" selection under the options screen. Hold A + X on controller two, and press the D-pad on controller one to change difficulty levels until the Gamer level appears.

Easy money:
Use a fully armed A-10 Thunderbolt for the green truck missions. Constantly bomb the trucks while firing.

Note: This trick is easier when used with a controller with a turbo feature. In the second level, defeat all of the land carriers, four small missile ports, and the front aircraft launcher. Leave the one close to the big gun intact. Position yourself below the weak point and keep shooting. Planes will rise from the launcher and be destroyed. This increases your money and pointsm giving you extra lives. On occasion the big gun will take damage when it drops down -- check your game occasionally to make sure the level did not end. Allow yourself to die after gaining an extra life, and repeat this procedure for more money.