Notebook
Many quizbowl players use a notebook to write down answers, game scores, clues to remember, or some combination thereof as they play games or practice packets.
Notebooks are cheap and easy-to-obtain plus players can add all kinds of visuals as well as extra notes outside of games when using the notebook as a study aid. A notebook allows players to easily flip through many days'/games'/tournaments' worth of notes, track their improvement over time, or otherwise ensure that they don't forget potentially-important clues as they go by.
Some players (such as Seth Teitler) will write down the answers to *every* tossup and bonus part as they go by, and perhaps keep running score beside those tossups. Others will merely write down clues or answers that seem especially pertinent to their knowledge gaps or areas of specialization.
Notebook users should be aware that some tournaments mandate that paper be blank if it is brought into a match, though most TDs and moderators are more lax about this in reality.