I stick mine on the back below about the tenth fret. Works well and doesn't get in the way of strumming. In the attached picture I'm using it and you can't even tell it's there.
updated by @phil-myers: 12/27/16 08:35:05AM
Short answer: as a Schatten Dualie user for 20 years now, both on guitar and dulcimer, it varies from instrument to instrument. You have to do some experimentation.
Where Ken suggests is a good place to start. I usually place mine below the lower sound hole on the right side of the instrument as I'm facing it. If the bridge isn't at the end of the body, usually sort of beside the bridge.
It's the best stick-on pickup you can get. I only wish I'd gotten another 15 years ago when the music store in Salisbury, MD, had them on close out.