Record all tenant repair requests. Make your number 1 policy (and follow through!) with tenants that all repair requests should be in writing. Have them send you an email with as much details and photos as possible. Your response should always be to reply as to when the repairman will be there. Then after the work is complete, send a followup indicating that you are aware that the work was done. File all your tenant emails in a folder so they are readily accessible in the event that you need to prove your case.