Quoted:
I do, but only when they have decent trade specials.
This. I did it a lot over the last few months to knock some $$ off of new games, but given that some of my older, less played stock is now depleted, I don't really have a reason to unless I buy a new game, beat it in a week or less, and have a reason to take it back. Or there's a good trade-in special for something I want to preorder.
Medal of Honor was traded in within a week. I beat the SP in a couple days, didn't care for the MP, and took it back to get credit for Force Unleashed II.
Of course then TFU2 sucked so I did pretty much the same thing, but on a shorter time span (and traded in my copy of TFU: Sith Edition to boot).
Currently I have the new Assassin's Creed, but I haven't been in much hurry to beat it (Minecraft sucked me in recently) so the value will tank on it since I didn't take it back within a week or two.
Not really anything coming out in the next few months that I want to rush out and buy so I'm not really worrying about this right now.
But like others have said, you can get better money by selling online. I just don't want to go through the trouble.