I had one rule. PAY YOUR BOARD ON TIME. That rule
kept the relationship between myself and the boarder pleasant. I had other
suggestions like don't feed someone else's horse. Try to exercise your
horse everyday. Feed plenty of hay. (We didn't have any grass.) Don't
monopolize the vet or farrier if you didn't call them out because the
person who did is paying for their time.
Whatever you do, put everything in writing. That
way no one can say they didn't know.