Practice, like a WHOLE LOT.
BMX / MTB we call em wall fakies or curb fakie -
ride up to a wall slowly, rock up on your front tire, pivot up and when you pivot back down start pedaling backwards and hope for the best. in BMX leaning back over the rear tire makes for better balance, but i dont knwo if thats do-able on a fixed gear.
rolling back is pretty easy with a freewheel, not so much with a fixed.
good luck -