Well, there are several steps here: 1)check to see what kind of game it is.
2)What does it add that is new to the genre.
3)Check a few scores, notably IGN and...well, here. Apart from the reviews wrote by Susan, John and Russ, I also try to take what Yahtzee says into consideration. If I like the franchise, I buy it no matter how much Yahtzee says. If it is about a new franchise and Yahtzee approves it, I buy it. That is not always the case however(hence, Brutal Legend which I will buy and COD4 which I didn't)
4)I try a number of stores to find the best price, which is pretty easy since I am in a small city.
5)If the game is subject to timed exclusivity, such as Fallout 3 when it comes to the expansions, I wait. Most of the times it pays of.
6)There are also seldom bargains, like the fact that I found Psychonauts for 6 euros. No-brainer.
7)I avoid sports, MMORPGS and point and click titles. Again, with some exceptions(Diablo II)
8)I never let expectations get the best of me. One of the reasons would be Spiderman 3.
9)Spoiler free. One example would be Fallout 3. It is an approximately 1 and a half year old game and I still do not know the ending.
10)Buying on launch is almost always a huge, huge mistake. Unless it is for de facto titles like Street Fighter 4, I wait for it.
11)If there is a game I like and it is both good and old enough for my PC to run it, again no-brainer. PC titles are always cheaper.
12)Lasting appeal. I am still playing RE4 and Burnout 3.