I've got a reasonable piezo pickup which I use for classical guitar and violin, which I affix with blue-tack. For contact pickups, they need to be in good physical contact with the wood or have something transmitting the sound through. Blue-tack is (according to a friend who does mobile phone audio stuff) actually one of the best sound-transmitting materials out there, so it's ideal. I don't know how well velcro would work - the pickup wouldn't be in contact with the wood so I don't know how well the sound would come through.

Contact pickups ("transducer" merely means something that picks up a signal) have to be in physical contact with the top of the instrument. Vibrations in the top of the instrument project the sound to the audience, and it's these vibrations in the wood that the pickup picks up, The pickup can therefore either be mounted internally or externally depending on whether you want it visible or removable, since the wood vibrates on both sides.

The pickup only produces quite a small voltage though (compared to magnetic pickups which are pretty meaty things). Compared to this small voltage, background noise in the cables becomes quite significant, so it's common to have a preamp at the guitar end (either in the guitar itself or hung on your belt) which boosts the signal, so that the noise is no longer so noticeable.

You can move the pickup around to find what sounds right, but once you know where to put it, fine adjustment isn't really an issue - the grease-spot left from the last time will show you where to put it. As for finding the right place, experiment with the entire top surface of the instrument. It's nearly always the most inconvenient place that sounds best, but that's life! My classical sounds best with the pickup on the bridge on the bass side, which is entirely the wrong side from what the instructions say, but it works!

Graham.