#1) Make games suck less, so that people want to keep them.
#2) Make games have good replay value, so that people want to keep them longer.
#3) Steadily lower the price of games after release so that you don't need to buy used to get the game cheaper.
#4) Offer free DLC to people who have already bought the games, while charging people who buy the game after the DLC comes out, thereby rewarding people who buy the games early, and giving them something extra for the $60 they paid, while the people who bought games later at $30-$40 don't get as much.
1 & 2 are the most important though. I sell games I don't like, and keep games I do like. Obviously not all tastes are the same, so someone sold Mirror's Edge while I'm going to keep it, and it can't be avoided entirely.