if you plan on moving up at any time, I would suggest also noting the stakes and situation of the note (FR/6max, tourny, etc.). I started doing that after I found some notes that could have been made a year and a half ago and were possibly no longer valid.

Usually my notes will consist of one or two memorable examples of play/betting patterns, and/or a strategy to be used to exploit the player.

Eg. TPTK is a goldmine, valuebet him to death, calls loose, bluff him, calls on 2nd pr-raises 2pr or better, etc.