Most people sign a service contract, file it in a drawer, and forget it until the car makes a noise. By then the copy they need may be missing a page, or the version they have may not match the version the provider uses. A little housekeeping now saves a lot of searching later.
What belongs in the file
Keep the fully signed contract, every page, including any addendum or amendment. Keep the terms and conditions document that the contract refers to, because the contract often points to it instead of repeating the details. Keep the sales paperwork that describes what you bought. Keep any receipt showing payment and any notice that the provider or seller sent you. If a sample contract is published for the provider, save a copy of that too, so you can compare the sample with what you actually signed.
Check that the copy is complete
Before you file it, count the pages and look for the signature and any initials the contract asks for. Confirm that the vehicle identification number on the contract matches the car. Confirm that the contract names the correct owner and the correct lender if one is involved. A small mismatch that looks harmless now can delay a claim later.
Record the upkeep as it happens
Many contracts require the owner to keep up routine maintenance and to show proof of it. Keep the invoices and the mileage recorded on each one. Store them in the same file as the contract, either as paper or as clear scans. The point is that when a claim asks for proof, you can answer within the window the contract allows without reconstructing a year of visits from memory.
Make the file findable by someone else
Tell a trusted person where the file is kept. If you are ever unable to handle the claim yourself, that person should be able to find the contract, the provider's phone number, and the reference number from any earlier approval without asking you. A digital copy in a folder with a clear name works well, as long as you keep a paper copy of the signed pages too.
Review it once in a while
Pick up the file at intervals and check that the provider's contact details still match what you have. If a notice you expected never arrived, the file tells you what to chase down.