I know that a Piezo crystal puts out a low charge when flexed, but what i don't know is if it continues to put out this charge or if the crystal needs to be continously flexed? Can you deform it and it will continue to put out a small charge or does it need to be continually flexed?

Quartz, the most commonly used piezo material is an insulator. When flexed electrons are moved and if the flex continues then the electrons will stay where they were pushed to, so there's a potential difference (voltage). As the quartz is an insulator the electrons can't flow anywhere to cause a current.

First example that comes to mind is piezo pickups - I have some in my upright Steinberger bass.

Forget any hope of 'DC' current output. As pointed-out above the intrinsic characteristic is a very, very high impedance output, which means you can't actually draw any useful current at all. Piezos will only develop significant voltage across a very large impedance - esp. if you want a decent low frequency response, as I do! (>10Mohm minimum loading via a jfet input, for the nerds)

If you can arrange that then piezos are very useful as sensors; they even have uses as micro-actuators if fed a high enough voltage!

Ok thanks. I just had an idea for something for a car that some of the chavs might like but I'm not sure it would work because of this.