Most service contracts ask one quiet question after a breakdown: did the owner keep the car up the way it was meant to be kept? The answer usually starts in the owner's manual, which lists the service intervals the manufacturer expects. Before you look at any contract, pull that schedule out and keep it where you can read it.
Find the schedule in the car's own paper
Most manuals have a maintenance section with a table of items. Each item is tied to a mileage, a time period, or whichever comes first. Some vehicles also show a service reminder on the dashboard that resets after a job. Copy the table onto one page with three columns: the item, the interval, and who did the work last. That page becomes your working record, and it is far more reliable than trying to remember when the last service happened.
Read the contract's maintenance clause against it
A maintenance clause usually says the owner must perform scheduled service, keep receipts, and produce them on request. Some contracts go further and name a source for the schedule, such as the manufacturer's manual. Read which document governs. If the contract points to the manual, your copy of the table becomes the standard. If it says 'reasonable' maintenance with no source named, ask the provider in writing what that word means before you rely on it.
Ask what the proof has to look like
Put these questions to the provider and get the answers in writing:
Must the receipt show the mileage at the time of service, or is a date enough? Must the work be done at a licensed facility, or can you do the work yourself and keep the parts record? Does the provider accept an invoice from a quick-service shop? Does the owner have to send records to the provider as they are created, or only keep them and produce them when asked? What happens if a service falls a little behind the interval?
Keep the paper the schedule demands
Scan each receipt the day you pay for it. Write the mileage on the paper if the invoice leaves it out. Keep records for every item the manual lists, not only the big jobs, because the small ones are the ones a claim reviewer is most likely to ask about. A tidy folder is a boring habit, and boring habits are what keep a contract in good standing.
When you compare providers, check how each one words this clause on the pages that publish a sample contract. This site's table of those providers is a good place to start, and the figures and terms it prints are the ones to test against your own schedule.