Request a digital copy of the checks or copy of the pay stubs and indorse with the lawful money verbiage. Keep the originals by scanning or taking a picture and send at least 3-4 proofs with one’s yearly tax filing when the time arrives. Keep a repository of physical AND digital proof.
NOTE: As an alternative, if one is "short on proof", a check can be signed to one's self and redeemed in lawful money using the correct verbiage. This still counts as valid proof.