That's been a couple of years now since I've stopped, almost universally, buying games upon release. As outlined above there are three main categories of games:
1) Nintendo 1st / 2nd party and 360 games with exceptional online features which will hold their value for at least 3 months and you can end up selling on for a minimal loss once you're done.
2) Heavily hyped titles which are significantly cheaper on the release date which can be sold on a pretty much break-even basis when you're done.
3) Everything else which will be under £20 within 3 months of release the resold with a minimal loss.
Another factor which particularly applies to single player games which suit several DLC packs is waiting for the GOTY edition to be released so that DLC can be resold instead of sitting in obsolescence on your 360 HDD once you eventually sell the game disc on.
I'm also now absolutely ruthless about selling on games once I've completed them. Used to have about 500 games until my mega-sale last year and I'm now looking towards my gaming shelf which contains 11 games...
2 of these are games I've worked on
Wii Sports and Wii Play which are nights-in-drinking essentials
4 are DS edutainment things which I'll get years of use from
That leaves 3 games that I'm actually playing or about to play