I keep a little notebook in the glovebox of each of my cars. I note date and milage of all service; like oil,lube,tires, and all parts changes.
My grandad tought my dad to do this, and my dad tought me. There is never any question as to when something was done or missed.
However I also put one of those little stickers on the windshield of the wifes car to help her remind me when the oil needs changed