Show up on time. Give the opponent a decent warmup (like don't blast balls when he's taking volleys... make the lobs relatively easy for him to overhead... don't blast returns when he's practicing his serve...).
Consider taking a lawn chair. A white towel to cover your head if it's sunny. Sun screen. Baby Wipes!!! Great for cleaning your hand of sunscreen or whatever else. Also, good for wiping sweat salt off your face and neck when a bathroom isn't convenient.