In order of importance:
I usually take into account first and foremost the synopsis of the game. By that I mean it's genre, background story, premise, and any indication of how the gameplay would unfold. If the synopsis appeals to me, it's a good sign for me to get the game.
I also have trust in some developers, such as Valve, Relic or Double-Fine, but never blind trust. I am more likely to take up a game made by them. The reason my trust is never blind is that I nearly lost my trust in some developers, such as Blizzard and Bioware since WC3 and NWN, respectively, which I used to trust blindly until an inevitable disappointment. I probably will have doubts as to buying a game from those businesses. EA is not one of those since I never had any trust in them.
In a similar fashion, I have faith in game series that I grew to like, such as Half-Life, Starcraft, Commandos (though they probably won't make those anymore), Baldur's Gate (all those rumors about a BG3 make me happy), and Dawn of War (DoW 2 looks to be awesome). I am likely to favor one of those games. Like my trust in some game developers, however, my liking to some series has been broken, such as Black and White, C&C (... *sigh*, EA, goddamnit), and Warcraft, and I'm far less likely to purchase a game from those series.
Finally, there's the price. Because I only buy games sparingly, however, I always have enough to get what I really want and know that I'll enjoy. So, usually, there's no problem with that. However, I'm sometimes likely to wait for a game to drop its price if it's like $60.