Go to Edit Profile hold L2, and then enter the following codes for new skins:
Dan Skin: LEFT, LEFT, LEFT, LEFT, UP, RIGHT, UP, LEFT
Nefarious Skin: UP, UP, UP, LEFT, RIGHT, UP, LEFT, UP
Duel Blade Laser Sword
With the game paused, input: CIRCLE, SQUARE, CIRCLE, SQUARE, UP, DOWN, LEFT, LEFT
Free Ammo and Health
Do a challenge like Annihilation Nation, start the challenge and quit. This bolt saving tip not only refills your health, but your ammo as well.
After you beat the game, you'll be able to purchase enhanced versions of the weapons you already own. You can only buy these weapons if you've leveled everything up though.
Fast Weapon XP
Play the Annihilation Nation challenge against Scorpio to get tons of XP. You'll have to vary your stages as some weapons, like the shield glove, need different stages to gain XP. For the shield glove, the battle missions in Marcadia are a good place to earn lots of XP.