Do you ever use sticky notes on your paintings? Sometimes I see problems that need to be addressed on an illustration I'm working on and if I don't have time to fix things right away I may forget the area that needs work. I've found that Sticky Notes come in handy at those times.
I'll quickly scribble directions to myself and stick the note right on the spot that needs fixing.
Of course you'll want the painting to be dry before doing this.
It may seem like an extremely obvious reminder to use but it works for me and I thought I'd pass it on.