As far as I can remember, I've never traded a game in. By the time I get bored with a game, or don't play it any more, it's practically worthless. Recently, I've started lending my excess games to friends (I loaned MW to a friend when I got Resistance 2 and Killzone 2, for example) if I don't feel like playing them. Then I can have them back whenever I want them.
Plus, I hate the way places like GameStop and Game rip off people who trade in.