I have a file cabinet that I keep all important documents in (house documents, insurance documents, medical documents, etc). I keep vehicle records in there too. Each vehicle gets its own folder. In the folder is all receipts for parts bought. In the folder is also a sheet of paper that has all maintenance and repairs I do to the vehicle. I write down the date, mileage, what I did, and part numbers for the parts I installed. When the sheet gets filled up, I start a new sheet. I also have a separate sheet just for recording oil changes.
I also do this for all my lawn equipment.