King Kong = 1000 gamer score when you beat game
Lost Via Domus (the tv game): 1000 if you beat game 100% (not that hard)
Avatar = 1000 if you get high combos (can be done in the first 5 min of the game)

I like to use blockbuster to rent/trade in games because they give more credit towards them. If I am done with a game I will trade it in (only if I know I won't play it any more) and I can get store credit for another game.