| Hi.
#1 Pay him/her enough.
I'm not kidding either.
When I was doing sound some 10+ years ago, there was ZERO appreciation towards soundpersons. We were the necessary evil, good for nothing freeloaders who got their money for sitting behind the desk, just listening and twiddling with the knobs looking like it did something. Everybody expected us to do everything and then some. Be the bus drivers, stage-hands, roadies, security, chaperons, bar-maids, instrument techs, You name it.
Make sure that this isn't the way YOUR possible soundperson feels, and maybe you'll be able to keep him/her.
#2 Pay them, did I mention that already? Pay them better than the bandmembers, after all, they're the ones that can make it or break it.
There's no motivation that drives a person better into that kind of "hobby" than money, as a matter of a fact that's about the only benefit there is in the small to medium scale band circuit.
A soundperson without a name and at least some equipment, ie. desk, processors, mics, pres. won't be getting near any decent equipment without a chaperon, if even then. Communication is the key, the house soundperson has to have talked personally with Your soundperson before the gig. Preferably several weeks in advance.
Regards
Sam | 
