If you're like me, you tend to sand drywall in fairly random fashion, sometimes in more than one room in a day. That makes it hard to keep track of (I think I get bored sanding the same wall and keep it interesting by moving around a lot, haha!). I've found the easiest solution to remember what has or hasn't been sanded or needs to be fixed is with a simple mark of a pencil. A small check means it's finished or a circle around a problem area that needs to be re-puttied. The graphite is easily hidden once the paint goes up and won't seep through like pen ink or marker.
Hope it's a helpful tip the next time you're drywalling!