I know this might sound extreme, but I actually use it, even on 3x3x3. I put fingers on each piece that has been memorized as I memorize it. Then I know immediately which pieces haven't been accounted for yet - they're the ones that don't have fingers on them.
It means my fingers are often in the way, but after a while, you get used to it and you learn how to hold them so you can still see the cube pretty well. Since you have a buffer piece that you don't need to track, that means a maximum of 11 pieces you need to put fingers on (for edges; corners are easier), but obviously you don't need to bother with the eleventh one, so 10 fingers is really enough to handle all of them. It also helps keep you from missing flipped edges.
This probably isn't good for when you're shooting for sub-15 memo like Ville and Haiyan (because it takes too long and the fingers are in the way), but I can get sub-30 memo times with it, so it's not that bad.