I keep a notebook and I manually write down every major repair or mod I do. (Oil changes, etc. are minor and I don't record those). I write down the date, mileage, and what I did in the notebook.
Later down the road, it will help me to remember when I did brakes last... when I changed the battery last... when those new shocks were installed, etc. And, if I ever sell the vehicle the next owner will LOVE my notebook.