I used to have kind of a problem with wanting to finish a lot of games, even going as far as to getting a majority of achievements, as kind of an OCD kind of tendency. Boy, did steam + disposable income ever cure me of that, fast. That said, I have kind of a 'system' that works great for me.
-I give every game maybe a 40 minute run, 10-20 if it's some shitty indie fodder deal. If it doesn't seem fun, I simply won't play it.
-I keep the main 'games' category as a catch-all for 'I haven't played this yet", and then sort it into a genre after I've given it a shot. If I ever get the burning desire to play it, I know where it is.
-Time is your most valuable resource, especially in the age of "10 games for $1!" bundles. If it seems like shit, move on. The way some people describe backlog clearing makes it sound like a chore or job, and thats pretty tragic when a game turns into that.
-I download maybe 2-3 'big' games every month, and don't download any more until I finish or quit those 3. I feel like this helps me focus on finishing stuff. (Gamer adhd was a great term, whoever suggested that!)
-I delete games as I go. Don't ask me why this helps, but it does. When I'm done with a game, I don't leave it sitting installed, otherwise the list of dozens of games at 85% complete on collectibles or whatever looks overwhelming
-I play quicker stuff first. Like I recently knocked out call of juarez gunslinger and it was maybe 5 hours, batman origins and that was 14ish. Compared to huge sprawling rpgs that take hundreds of hours, I kind of knock out shorter stuff first when I can.
My backlog is always at around 5 'big' games these days, with a few perma-installed games like civ5, xcom, counterstrike, and killing floor. I keep it about there, and only buy new stuff if the deal is insane, or something I really want early. I have like a million games that I could go back to and play again on a harder mode, or do different stuff in, so I just see it as I'll never run out of stuff to play, ever. Realistically, I feel like I'll always have 4-5 games ahead of me to play, and thats alright with me.