I have recently had a piezo pickup added to a nylon string guitar. There is no onboard preamp, just an undersaddle piezo running to a 1/4 jack in the bottom of the guitar body.
I noticed a problem with 50 Hz hum, and through a process of elimination, have narrowed the cause down to my own body. When I touch the jack, a pedal on my pedal board, a metal electrical item that is completely disconnected from my rig, or even a water pipe, the hum is eliminated or reduced (good video on this here).
I've looked at anti-static wrist straps like this one (I'd attach the clip to the jack plug), but surely there has to be a better solution than this?