Well, on Steam I've sorted my games using the categories feature, and backlog games end up under the category "[Games to Play]" (the brackets force it to the bottom of the list). Generally if I'm in between games, I'll just start trying stuff out from the backlog and see if anything connects. Usually I try to give games at least 2 hours, and by then I know pretty well whether I'm having fun or not. Generally the longer it sits in that list, the less likely I am to actually enjoy it.
I was in a similar situation with Rage specifically, it was just sitting in that list for so long. Finally I was in the mood to play a shooter, and in less than an hour I had decided I hated it. Just like that, a 15 hour game that had sat in my backlog for two years was gone. Things always end up in there with the intention of getting my money's worth, but the reality is after enough time has passed I just don't care; I've moved on. Periodically I just sort of reconvene with myself and consider two things: what am I playing now, and what do I want to play next (without looking at what I have available). Then I'll go to my backlog list and just start to purge things. Admittedly, most of the time the games I never play are from bundles, Humble or otherwise, so they're not hard to purge; I'm not impervious to bad Steam sale decisions, but those certainly aren't the reason why my library is so large (I'd swear about half of my Steam library consists of bundle games). Console games I tend to feel worse about, and they're not quite as easy to hide. I should really be playing that copy of Drakengard 3 I bought, but now it's looking like I probably won't get to it until February with all of the other things I intend to play before it.
At some point, you just need to be realistic about what you're likely to play and what's worth your time. If you have a large backlog and games aren't fitting into those categories, cut your losses and stop worrying about them/get them out of sight; if you really want to play them in the future you'll find your way back to them.