I keep a small journal. I write the date, mileage, and fuel level in the journal for each entry. Then I detail what maintenance was performed, and when the next time that maintenance should be done. I also write other information such as part numbers, manufacturers, or any other important information regarding a repair or replacement so that I can reference back in the future if I need to do it again.

I am a professional mechanic, so its not a big deal for me. Just like making a repair order for my own vehicle, really.