Chetan Vashisht
Oh thats easy, use your knuckles. Use your left hand and count with your right. Starting from the left of your little finger knuckle, start moving right. You should be able to move 7 times before hitting the end. Once you do, just start again for the left. Every time you hit an upward bump it signifies 31 days, otherwise it's 30 days or feb.
That Man
Hah! Old school method :) learnt this as a kid and always took this for granted thinking this is a fairly well known technique (which I still think, probably is)