I've used several types of acoustic PU over the years. The worst I have come across have been the contact mics that you stick to the body, they are horrible! The best have been the saddle Pizo style iamthe_eggman was on about. Im using a saddle pizo at the moment and I really like it.

Another option is the good old SM57 or a condenser mic. Careful positioning can yield really nice results however it means no moving about !