I make lots and lots of notes. It's amazing how many people do the same things over and over.
Recently I had a note about a person that read "Very, very sticky with under-pairs". I assumed (correctly) that was the situation on a flop where I had a large over-pair. I shoved, she called (as expected and hoped for), then she made a set on the next card. Not the hoped for result, but the note allowed me to make an educated decision that was going to make me a tidy profit 90% of the time.
Another very simple one is: "Very Tight". Lots of players are very tight, but not all. So knowing exactly which ones are in advance of a play can be very helpful.
"Very Loose", "Shoves a lot", "Slowplays a lot", "Very sticky in blinds", "Limps w/ AA" - these are meaningful notes that can't always be easily gleaned from a HUD, assuming you even use one, which I usually don't.