for me the best way is to trade games in regularly, while they are still fairly new and worth something.

I only have around 5 games but have owned around 50 or 60. As soon as you finish a game think will you ever play it again or will you miss it ? if no then trade it in

Also, puy old pre-pwned games to keep your costs down, and every now and then pick up a game of a type you wouldnt normaly get, you often get some pleaseent surprises - a change is as good as a rest