Take two weeks off, then quit.
Sorry, I had to be the first one to throw that out there, lol.
A few people suggest time off as a solution... and maybe that worked for those people. Personally I think that's a pansy way out. If you enjoy the game and want to get better, you can do it at the table better than away from the table.
I think part of it may be that when someone talks about having a bad streak (especially a long bad streak) they are thinking incorrectly. They are comparing their usual game to the game they wish they had... the game they have one day a month.
Maybe you don't play "play bad" for long stretches because you're in a slump, maybe your overall level of skill is just not as high as you think it is. Maybe those slumps are you shooting normally and the rest is you shooting unusually well.
I know... hearing that might make you think I'm being a díck or that I just don't know you well enough to say something like that. But I truly think this is a common problem that causes people to say "man I've been hitting 'em bad like all week!"
The single best cure for not playing well, whether it's missing every ball for a month or for a single missed shot ... is to educate yourself on what you do wrong, learn stuff you didn't know, and spend time practicing this stuff. You want to reach a point where you can do it properly at will, no matter what kind of day you think you're having.
I personally find that I shoot better after watching pool videos, either high level tournament matches or instructional videos. Especially instructional videos because for a little while I am going to focus on what was being taught (even if I already knew it) and that extra focus shows in my game.
Watch videos, focus on improving your fundamentals and stroke during every shot (forget about whether you win or lose the game, just focus on making each ball and getting shape) and keep a realistic, positive attitude. Don't get pissed if you go for a breakout and come up dry, or miss a really tough shot. You're supposed to miss it, that's why it's called a tough shot =) Basically don't be pissed at anything except when you choose to shoot carelessly instead of taking it seriously.
Although it might sound boring, you might even try some drills. And change up the game. If you play 8 ball all the time, try some 10 ball for a bit.