How do I get my NES games to work?

I have about 20 old school Nintendo games, with an NES, and every so often, I enjoy going back to my youth, playing Super Mario Bros. and Duck Hunt, among others. One problem, though: whenever I pop in a game, 90% of the time, all I get is a flashing grey screen. Rarely have I ever gotten a game to work, and only once have I gotten a game to work without being all glitched ( Teenage Mutant Ninja Turtles, for those curious). I have that classic grey box, and between it and the game cartridges, my lungs are dangerously close to giving out. Does anybody have any ideas? All my games have worked just fine in the past, save for Hoops, but lets face it, sports games on NES suck, am I right? The answer is yes.