using Space Marine as an example i'll give ya'all a basic run down
1)contend based on announcement trailers: assuming i hear about the game this early, this will dictate how much attention i pay the game, in this case, the trailer was 'fucking awesome' so i kept an ear out for more info.
2)press releases from the devteam or publisher: more details about a game are always welcome, and can sway weather i get a game or not. this is all press releases, be it in a magazine, or a video.
2.a)dev teams attitude toward the fans: i added this purely because of ninja theory's attitude toward DMC fans, if your team gose out of its way to disrespect, and BLAME for the failure of another title, i will not buy the game and interest ends here.
4)Demos: this will ether save a game i'd lost hope in but was still semi interested in, be the final nail in its coffin, or make me start working around $60 I've suddenly lost due to over exposure to awesomeness.
X factor) Good Will: has the dev team, in the past provided me a good gaming experience? how many times have they done so? this is a big one for me, and can take a lot to over ride, this is usually what i base 99.99% of my CD purchases on. (i pay less attention to music these days)
the ending result is: i pre-ordered Space Marine and since have enjoyed it a lot.
its not fool proof, but serves well enough