Not sure if anyone mentioned this but there are 2 great tricks for getting better sound out of piezos.
First method: solder them to a mono jack in the usual way, then use a DI box. This alters the impedance and provides a good mic-level signal for any mixing desk.
Second method: some piezos have a two-part zinc contact in the middle. If you solder these to a balanced mic cable like this:
small contact = +
big contact = -
copper disc = screen
Stick an xlr on the other end and use any mixing desk or mic pre. Because the signal is sort-of balanced you can get away with very long cables without interference.
Or buy one of Jez's mics, they sound great. Piezo foil is nice too but you have to get that from the states these days afaik. xgus