Stop buying stuff just because its cheap. Instead, ask yourself if you see yourself playing it anytime soon (ie, is it a game you really want to play over the majority of other games in your backlog). If its a no, leave it.
For example, I saw the Serious Sam Collection the other day for $10. Usually I would have picked it up because its $10 and the games aren't bad. But, it's not a game I would prioritize over other games in my backlog, which means it would just sit collecting dust. So, I left it behind. $10 isn't much, but I would rather put that toward a game I really want, then to buy a game I'm not really interested in.
When it comes to playing games, I had a method where I would play one of my oldest games first, but that just made it seem like a chore, like I was just going through the games to get through them. Now, I just pick whichever game I want to play the most, whether its a newer game or older one. Because I have a huge backlog though, I try to make that choice easier by focusing on one genre (e.g, rpg, 1st person, 3d person, etc), then picking a game from there.