Count to 31 on one hand? No problem, if you know binary. Watch the video.

Or you could count to 1023 on two hands. Or 1048575 with hands and feet. That would be a rather boring video though...

Basically, the trick is to decide on an order of your fingers/thumb. Then move the first one up/down every count, the second one up/down every second count, the third one up/down every fourth count, the fourth one up/down every eighth count, etc.

There's a detailed explanation of slightly different methods at Mr Speaker and intuitor. Wikipedia calls it "finger binary." The oldest known web source for this seems to be lyzrdstomp.