beware that some piezo's can generate large voltages !
this can happen when they are dropped, or knocked on.
sometimes being successful is the most dangerous ;-)
seriously, be careful what you plug the piezo's into.
In my opinion as a piezo tinkerer, if you want more
sensitivity, use a better amplifier.
I assume you know about the knock/blow/sound sensor circuit ?
this is one version, with a very nice design example spelled out : http://www.nerdkits.com/videos/sound_meter/
another good source of piezo amplifiers for audio range is to google for "piezo pickup".
Guitarists know all about recording audio with a piezo disk.
the piezo disks can be seen as capacitors (with varying capacity) just like electret mics.
This explains why connecting them in series will not have much effect.
btw, those little electret capsules already have a jfet amplifier
on board. you need something comparable for your piezo.
one other way I *believe I have understood
* to increase sensitivity a bit is to pre-stress the piezo disk.
No idea how you would do that in practice though. the piezo stuff is a ceramic and brittle !
If you NEED to bend the piezo, e.g. because you want it attached to a curved surface, check out
PVDF piezo. Sparkfun has those, but they require even more amplification.
in all cases, let us know how your experiments went..