Schertler ACC-PUT-1

This is the ONLY thing you need to stick a piezo/contact microphone. Search no more, this thing is so amazing:

1) Stick well
2) Non greasy (doesn't leave any marks on the instrument)
3) Respect the (fragile and soft) varnish of delicate instruments (violin, classical guitars, etc)
4) Has the right balance between softness and hardness to conduct very well the sound emitted by the instrument.

In short, it is blue tac combined with children putty SPECIFICALLY MADE for varnished and fragile instruments (Schertler is a mic and preamp maker for (bow) stringed instruments).

I made a DIY piezo mic for my violin. I sticked it under the bridge, against the body. I can reposition the mic as much as I want and need (particularly handy when I go to the violin maker and he asks to remove the mic). In about a year, I never had any greasy marks, nor any deterioration of the varnish.

The only cons is that it is expensive ... BUT you can use it for at least 10 piezo microphones (so this makes it 1.59¤ per pickups ... not bad eh!?). And frankly, paying for something that made me so happy and proud, just makes it worth the investment!