[NeoTar]

The number of days in a month sits in a nice space of complexity for me - it's hard enough to be non-trivial, but simple enough you feel you should know it.

Probably the easiest thing to remember is that short and long months generally alternate (the exceptions are December/January, and July/August).

I like the 'use the knuckles of your hands' technique - put your hands next to each other, and count the months using your knuckles and the spaces - so January (knuckle - high so long-month), February (space - low so short-month), March (knuckle - high so long-month). When you reach July, you get your left-and-right knuckles next to each other, so two long months in a row. [EDIT: I can now see that I'm about the fifth person to mention this!]