This is probably the most obvious thing we mark in our music and very important for muscle memory. The quicker we settle on the fingering that works best for us (there’s usually a few good fingering options in music passages) the better.
Marking repeated patterns
Use coloured pencils to map out repeated phrases. Do this particularly while learning certain J.S. Bach pieces!
If there’s a jump or something unexpected coming up, particularly on the next line or before a page turn, draw a small pair of eye glasses right beforehand so your eyes will look ahead more quickly than usual.
Writing out chords
The most common chords in a key signature are its I, IV and V chords. Look for these (either broken or blocked) and write out chord symbols near where you find them. Find uncommon chords or key changes as well. Knowing you have gone from, say, C major to a minor for a few measures will really be helpful as you practice, and a simple pencil marking can serve as a reminder of what chord or key you are playing in.
While memorising a piece, write the date of any memory slip-ups. Have someone listen to you, following along with your music score, so they can make these markings. Play for someone several times every few weeks or so and see if your memory slip spots are recurring. If so, practice extra in these tricky places.