Trade them online, there are plenty of cheap and free trading sites:
www.gametz.com
www.tradegamesnow.com
www.goozex.com
Or sell them on ebay/amazon and pay their high fees, either way you come out far, far ahead of trading things into GS.
A while back I realize the only thing my collection was gathering was dusk. the majority of games lose most of their value within a year, so if you can force yourself to trade off games after you're finished with them you can frequently get something else (or cash) that you also want to play.
Basically for every game I buy new, ($50ish since most big 360 release are avaible at Costco for that price) I can resell them for around $40 within a couple months of release ($30ish profit on ebay after fees) or trade them for another recent release ( Like Mass Effect for Halo 3, or Uncharted for COD4) and in the end I've either got about half my money back or a new game, which I can also trade off when I'm done.