So I buy and trade in a lot of games.  The key is to realize how long you can hold onto a game and still get a good value back for the game.  In this example I returned four games to GameStop and I have an Edge card.  I decided to trade in Call of Duty 4, PGR 4, Need for Speed and Kingdom Under Fire all for the Xbox 360. 

Here is the breakdown:
Call of Duty 4 -  38.00
PGR4 - 18.20
Need for Speed - 16.00
Kingdom Under Fire - 27.00

Total - 99.20

Now I played these games and can say I know for a fact I would most likely never play these games again.  I tend to keep very few games and only currently own 15 games.  The savings I got from trading in these games allowed me to buy Devil May Cry 4 and still have over $34 in store credit. 

Some people really like to collect their games and keep them for display.  I do as well but I only keep games I know I will play again.  It's hard to decide which games are good enough to keep forever so I often wait till I have several games to trade in.  By then I know if it's a keeper or a dust collector.