Pause the game and hold all four shoulder buttons and then enter one of the following codes, depending on which character you are playing as. Can be used as many times as you want.

Up, Square, Triangle, X 1000 Experience for Aragorn
Up, Triangle, Up, Down 1000 Experience for Frodo
Circle, Triangle, Up, Down 1000 Experience for Gandalf
Circle, Circle Triangle, X 1000 Experience for Gimli
X, Triangle, Up, X 1000 Experience for Legolas
Triangle, X,, Down, X 1000 Experience for Sam