My father's method was writing the odometer reading in pencil on the top of the engine after each oil change. The "head" method doesn't work so well, unless your memory is infallible.
For me:
1. Paper logbook in car, with odometer reading and date of each fuel purchase, repair, and maintenance item. Shows quantity and price of fuel, name and brand (but not price) of parts and fluids. Did this for every vehicle I've owned, from the beginning of time.
2. Spreadsheet of repairs and maintenance. (Based on #1 above, with a little additional info.)
3. Spreadsheet of fuel purchases, including running average mpg and ratio of actual mpg to that claimed by dash display.
4? I started logging maintenance in ToyotaOwners.com, but have about quit, because it's very time consuming, will not be visible to any future owner, and ---strangely--- is apparently not visible to the dealer service people either, so it seems a pointless waste of time.