Greetings and a Warm Welcome to our TorqueCars Forum my Friend!
Good to have you along with us
Well I have a service history book that gets stamped by the garage concerned; it also has a note of what was fitted and the mileage recorded that the service was pertaining to.
Note pad in your phone is pretty good for this, new record for each car or use a Cloud hosted spreadsheet which can be updated from any number of locations.