The first thing I'd do, though it's a pain in the ass, is make a list of all the games you own.
Then, look at all the games you own and rank them A-F through how much you want to play them, I also keep an S rank (above A) for stuff that has sentimental value or is one of my favs. Then rank each game based on what you know, or if you're unsure gameplay pop it in for a bit or browse online. For me, anything that was in the C range or below I felt fine getting rid of.
Granted, then you have to go through the process of selling stuff, which is a lot of work!