If you can stand it, get a guitar that you're not in love with. My "baby" is an older Guild that you'd have to pry from my cold, dead fingers. So, I got a cheap Alvarez--it still sounds and plays OK but if you sit on it, I won't have to kill you. Also, it LOOKS cheap--no frills, no trim--so nobody is ever itching to play it just for the sake of touching it. I know that sounds perverted, but a friend of mine once had a gorgeous old Fender and everyone was always bugging him to lend it out just because they wanted to say they'd played this wonderful guitar.
Failing that, I just tell them "no", unless it's someone I know & trust and they're mostly sober. Ditto for the lap dulcimers, which really are irreplaceable since their builder has retired--I can only think of one or two people I'd let play them. (On the flip side, I would never ask somebody if I could borrow their instrument unless they knew me very well and knew I would treat it with care.)