Fuckin' mk decided he doesn't take a shower until he's done 20 pushups. It's so devilishly simple that I had to do the same. Which means I did 20 pushups on Sunday, 10 yesterday, and 6 today. Ouch.
This is my experience, too. After taking a long time off, I can do about 12 clean push-ups. After a week or two, I can do 20, and after two months doing 30 feels as easy as walking down the hall. It's definitely worth sticking with it.
It may help to make them easier for a bit. You want to be getting close to your goal reps (although 20 is already a ridiculous number). Try putting your hands on the edge of your tub or the toilet, or even counter (be careful though). This decreases the weight on you hands (which I've found for normal pushups to be about 2/3 bodyweight).