Polystyrene is generally considered to be one of the best dialectric materials...second only to teflon in terms of dialectric absobtion. It's tough to solder though, as it doesn't like heat, so you need to be quick with your iron.
I'm not sure about how polystyrene reacts at high impedances like right at the capsule section of a mic though. I thought that mica was a pretty common choice there...but is a worse dialectric overall than polystyrene.
Cheers,
Kris