I can usually remember the important things for a few days---I'm obsessive enough about the brewing details that I usually remember the important details and measurements for a few days. However, I try to keep notes just in case, so I always print out a recipe sheet and stick it on the fridge. As I add malts to the grain bag, hops/extract to the boil, etc, I check them off. I also keep notes on the sheet, noting actual mash temperatures and volumes, any "game-time" modifications to the recipe, etc. Then whenever I get around to it, I write these up neatly in my log.
So my brew day notes are generally very complete. It's the other stuff that gets sketchy. The worst is on brew days, when I almost always forget to write down the priming sugar quantity. I also like to keep qualitative notes on the fermentation progress and record any temperature changes, but these tend to be hit or miss.