On all these cheats you MUST pause and be holding the L and R buttons the whole time.
Aragorn:
Up, B, Y, A, Give Aragorn 1,000 Experience Points
Y, B, B, Y, Replenish Aragorn's Arrows
Up, B, Y, Down, Aqcuire four upgrades immediately, when leveled up high enough. Upgrades: Dark Deliverance, Shield Cleaver, Lightning Strike, and Swift Justice.
Legolas's Gimli's and Sam's cheats can be used more then once.
Legolas, Gimli, and Sam:
A, Y, Up, A, Give Legolas 1,000 experience.
X, X, Y, A, Give Gimli 1,000 EP
T, A, Down, A, Give Sam 1,000 EP
Gandalf:
X, Y, Up, Down, Give Gandalf 1,000 EP
Y, Up, B, A, Bring Gandalf to level 4
Unlock Frodo:
Y, X, X, X, Unlocks Frodo for use.
Y, Up, Down, X, Brings Frodo up to level 2
Secret Characters:
Frodo
Merry
Pippin
Faramir
You have to have beaten The Cracks of Doom to get, Merry, Pippin, and Faramir.