I move my fingers (and hands or a prop wand if I'm carrying one) to "write" stuff in the air when I'm doing serious thinking. The way that helps me is that I can keep more thoughts in my head. This doesn't (just) apply to math problems (since I hardly know any math and can't do much calculations in my head). My current hypothesis for why this works is that it couples certain actions to certain ideas and repeating the action makes it easier to recall the idea. If I'm right about that it might be learnable and useful, to a similar extent as mind palaces. By coincidence, I've been thinking about trying to formalize this technique in some way since Saturday.