We all have that friend who seems to buy every major release. They always have the newest games and usually within a week of release or sooner. Why not use those friends to maybe land some deals? Now this tip is not guaranteed to work as much as the others. This one relies heavily on your relationship with these people and your communication skills.
I had a friend who purchased Mass Effect 2 on day one. Having the beat the game within two weeks, he was bored of it and was seeking to trade it. He offered to sell it to me for $20. I accepted. He was perhaps more desperate for money and willing to deal, but the principle still applies.
Gamestop gives cash, but at a reduced price, and credit can only be used to buy things in Gamestop. So by making an offer or being approached by a friend like this, you can offer cold, hard cash or maybe even trade titles. Never underestimate a gamer who has grown bored of their purchase.