If they're games you're pretty damn sure you won't be playing again, go for it.  If they are games that you can easily see yourself picking up and popping into the console again sometime down the road, don't do it - you'll regret it.  My other piece of advice would be not to use whether you beat a game as a measure of whether you should trade it in.  Still ask yourself if you might play it again.

I'm kind of in the same situation. I decided about a year or so ago that I was going to stop trading my games because I always regretted it. Recently I've been in desperate need of cash though and thought about selling some. So far I've gotten by without having to sell any, but when there's games that haven't been played for months on my shelf, it can be tempting. I just know I'll regret selling any if I end up doing that though.  *sigh*

I sell on ebay for higher profit, but if you have older games like that you're probably not going to get much either way. If you can trade em in when your gamestore has some type of promotion where you get extra credit for trading in games.

You're far better off selling most games on ebay. You'll get much more money for them. Also, you might as well not bother with FIFA 07 and PES 6. Old sports games are worth next to nothing. You'll probably be lucky to get £0.50 for those.

I sell games on eBay or sell them at my town' Reuse eXchange (ReX).
 
Why should I take a game that GameStop will sell for $48 and sell it to then for $20?  If I sell it on eBay I get $35 to $37 for it, I still get more cash or credit then I'd get at the game store and it is nearly as easy.   GameStop makes its easy, but WE pay for our laziness in that exchange.  Gamers who buy from Gamestop or storses like that are creating a system where GameStop, who doesn't make a damn thing profits a few times for each new game sold.  Is Activision, Ubisoft, EA or Namco sleazy?  Sure, but at least they DO something they finances & publish games we want?  What does GameStop, Wal-Mart or any of the other "trade-ins" stores do?  They just sell crap at inflated prices and they buy it back at reduced costs only to sell it to some other poor shmuck for an even higher profit.  I'd take a publisher's sleaze any day because at least they are an actual part of what makes the big games I like possible.
   

i had a rule where i never traded in games or anything. even if i wasnt sure id every play them again, i know a.) my friends might be interested in playing them, or b.) id eventually get hit with a nostalgia wave and desire them again.  but this summer i traded in a bunch to finance my purchases of the games i wanted this fall. now im about two clicks away from buying half of them again. trade in regret, good sir.