When I'm not playing a recent release (which usually jumps to the top of my priorities), I have a list of games I want to play and use Random.com to pick one by chance. Works pretty well, actually.
That sounds like a pretty good idea, I might give that a go as I undertake my back log. Unless it's a new release I'm really excited for, I tend to take 2-3 games out of my shelf that I want to play, and choose which of the three I want to play the most, and then play my second choice after completing the first game and so on. Or it's totally mood based and one day decided I just wanted to really play something like SSX or Fist of the North Star and proceed to do so.
I decide first what type of game I want to play. Genre, story-rich, gameplay focussed and all then. Then I cross reference that with my library. I keep a document of all my games, owned and wanted and tagged them by genre and major/minor concepts. I just start searching and something interesting will pop up in seconds.
Though... I kind of feel like a nerd now for even having created that document. But oh well.
I only play 2 games at a time and try to have some sort of opposite balance/difference between the two. I always choose one that will be difficult(always play on the hardest difficulty) and stressful and one i can just sort of jump in and relax. I also never play back-to-back genres to keep things fresh. IDK, that seems to keep my hobby fresh as opposed to the same stuff all the time. Currently playing Batman AO and Ghost Recon Phantoms.