Coelocanth: So I go with the $5 rule: if a game is $5 or less, I'll purchase it, assuming it's a game I want, even though I may not have the time to play it right now.
That there is the reason we have big backlogs! So many games are in that price range during sales. This is why keeping my mind on the total I've already spent on a whole lot of games I haven't the time to play, helps me control the impulse buy reflex.
I've given some thought to the speed with which I finish games. I have around 100 on Steam and 61 on GOG (and let's not forget, some of those games are gold editions with included expansions or DLCs, so the number is really higher). Even if I don't finish said games due to boredom or repetitiveness, they'll still eat a lot of my time. I also have a job, chores and a wife to spend time with, so realistically speaking, how long will it really take me to go through my current backlog? If newer or older yet better games show up on the horizon, they'll get installed before the not so good, heard-they-were-good, I-bought-it-on-a-whim-because-they-were-cheap ones.
I wonder if I'll still have XP or even Windows 7 by then. I wonder if my unplayed games will even run five years from now (I can expect GOG to keep up with the times, but not Steam to make it's older titles playable). Then those $5 will feel like a waste, and worse, it wasn't just five dollars. It was $4.99 + $7.50 + $2.99 + $4.99 + $3.75... That's why the big number helps me rein the wallet in.
I'm seriously trying not to buy games this Christmas. I have enough. As soon as I no longer have enough, I'll indulge myself with a treat, but right now, I'm stuffed.
Curiously enough, this hasn't stopped me from trying to get free games. :-/