If you are engaged in an energy harvesting project that involves piezo sensors powered by vibrations, you might want to check out the most recent issue of Applied Physics Letters (Applied Physics Letters 97, 014104 (2010)). In it researchers from the University of Lyon in France describe a scheme for boosting the output of off-the-shelf piezo sensors by a factor of 20 when applied in energy harvesting. If used with special low-loss sensors, the improvement could be a factor of 40.
The idea is to feed back a brief portion of the electrical energy that the piezo element harvests. The feedback interval is small with respect to the vibration frequency. This sort of feedback seems to provide a push at the piezo sensor's resonant frequency and so tends to enhance it's output.
However, the researchers' work at this point seems to consist entirely of modeling the effect. They don't mention verification of their technique through physical circuits.
You can find the Applied Physics Letters article here: http://apl.aip.org/applab/v97/i1/p014104_s1?isAuthorized=no