If your bouzouki is the type with a metal tailpiece where all the strings attach, he easiest way to get rid of the hum is to run a wire from the tailpiece to the nut on the jack socket so when you touch the strings you are the ground. It will still hum when you let go the strings. If the pick-up is one of the ones you install under the bridge saddle and you installed it yourself, it is very important to have both the bottom of the saddle and the bottom of the saddle recess absolutely flat so that the pick-up is crimped equally all along its length between the saddle and the top of the instrument. If not, you will get weird results and uneven reproduction of sound freqencies. As another approach, try plugging it into a Direct Box which has a Ground (earth) Lifter before sending the signal to the amp. DBs are expensive - take it to a music store and see if it works first.