Years and years of practice! hehe..

I have been playing this game since MK1, in the arcades, and some of the moves are easy to remember, and most characters share the same button combinations. It's best to pick a character and learn his moves, then move on and learn another character's.

For example, I think Sub Zero and Noob Saibot has pretty much the same button combos, so most of them should do another special move if you play with the other character.