So you are doctor, a repairman, a maintenance person, a car service technician

Here's my beef... I have a car, a motorbike, a gas installation, a water filter and some other staff. Whenever I get someone over for service or maintenance the thing ends up with: "Oh and don't forget to call me up in six months time for maintenance" Yeah riiighttt, as if I didn't have enough on my plate, now I have to remember something six months from now. I loose track, they loose clients or at least they don't get as many gigs as they can. So now I'm building a simple service for one of them to use in the field, just to make sure my gas doesn't blow up in my face. How do you make sure that your clients are informed when the time comes?