I keep a big list of games of potential interest and continuously research them. Over time a few bubble to the top (in terms of my interest).
For research BGG is invaluable of course, especially the game-specific forums and comments. I take ratings into account but with a big grain of salt. I read the rulebooks, watch videos too (playthroughs are the best but reviews too). If there is a good online implementation (especially BGA or Yucata) I definitely give that a try. If there is a TTS module I load that up too even if I don't have anybody to play with, it can be a good way to examine the components.
Research can be almost as much fun as actually playing :)
In short I have to be excited about the game, I have to have some level of understanding of how it plays and the price has to be right.