I can boil down my game-buying methodology to one sentence "Is it less than £10 on Steam?", but that would be a little _too_ simple. The actual decision-method is closer to "Is it less than £10 on Steam for a mainstream game with great reviews, or £5 for something Indie with great reviews, or £3 for something quirky that looks like it might be worth playing about with?"
When I posted yesterday that I was picking up Fallout 3, and whether it was worth picking up Morrowind and Oblivion for £5 each on top I received multiple messages from people saying "Buy Skyrim instead!", which to me felt like the height of pointlessness. I've already said that it's FO3 that I really want, and Skyrim is (a) a very different game, (b) twice the price and (c) still getting regular patches so that
the dragons fly in the right direction.
People seemed to vie in actively putting down Morrowind (apparently so old-fashioned that one would need a monocle _and_ a penny farthing to play it) and Oblivion (so repetitive as to be unplayable), despite me remembering that when they came out people raved about how awesome they are. And, indeed, if I check the reviews they both get Game Of The Year awards from multiple places, 90-95% reviews, and general acclaim. But because everyone has now moved on to _this_ year's games, they aren't worth having.
As someone who has recently had huge amounts of fun playing Cave Story, a game that looks like this:

I can quite happily say "Fuck That".
While I am quite sure that Skyrim has fixed some of the things that bugged people about earlier games, and contains all sorts of shininess undreamt of back in 2002, it seems to me that people really enjoyed playing Morrowind, have produced all sorts of addons for it, and that if I can't get £5 out happiness out of it then I will be very, very surprised.
So, going back to the actual title of this post - my method is not to be so caught up that I can't wait a year for a game to drop in price, have a massive backlog of games that will quite happily keep me going* for numerous months, and realise that I will never play every game that I want to, so I will happily browse away at the massive amount of options that I have. Heck, I waited more than six months to play Portal 2 when it came out, and that was fine too (I even managed to remain unspoiled!).
*I'm halfway through Dragon Age, and was very-much enjoying it when my life imploded and I didn't have the attention span for it, have Mass Effect 2 and DA2 after that, then Fallout 3 to play. In the meantime Cave Story+ is my active game, with Defense Grid as a background snippet game, for when I have 15 minutes and want to pick up another medal. Batman:Arkham Asylum is after that, and then Bastion. Plus Julie and I had Sacred recommended to us as a fun co-op action-RPG. Oh, and I have non-gaming things I do for fun too.