Written observations recorded during or after a track session. Session notes help you track progress, remember coaching advice, and maintain a record of what works at each track and in each car.
Bentley advocates for this in Ultimate Speed Secrets: "A driver should keep a record or log book with the details of each race, practice, test, or qualifying session. Use these records to learn from, looking back at them when returning to the same track again, or when you are having a problem with a specific area of your driving." He describes his own practice: "I like to write down the objectives for each session, what driving techniques or plans I need to use to achieve them, before going on track." Over a season, these notes reveal patterns — perhaps you always struggle with the same corner in the afternoon when the track is hot, or your consistency drops in the third session of the day.