I pre-order games that I'm looking forward to and have nice bonuses for doing so. So far I have Dead Island, Sonic Generations, Gears of War 3 and Rage pre-ordered. I mostly pre-order stuff online, because they have a nice habit of posting it out so I get it a day early. Which is nice.
If I'm not looking forward to it or I don't think it's worth full price I'll either buy it when it gets cheaper or rent it, if I think I can finish it in one go.
What I've been doing recently as well, is buying the games new, completing them as much as I want, then trading them in to get money off the next new game. Sure, I paid £40 for the first one, but then I'm only playing £10-15 for each additional one.
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Still the number one reason for the success of the Internet.