My head.
Actually this forum has become my record for me, I tend to post when I do stuff (like cut open a filter, I have the date and post the mileage in the thread).
Before I went to 5000 mile oil changes regardless of time, I wrote in on the wall of the garage (date and mileage).
Transmission fluid- every 10,000 (on the 10,000) on the truck, the Scion will be every 30,000 (started at 30,000).
Fuel mileage on the truck is tracked on an app on my phone (and I do put other maintenance stuff on that, but not always), I don't track it on the Scion.