Yes chelle covers this topic well Polarisers are an effective way to reduce the luminance range in a scene.. They work as they allow light to pass that is vibrating in one direction only blocking the light from all the others, works best when the sky is already blue. They work best on an SLR where you can see the changes in the viewfinder.. Modern ones should be of the circular type.. Dont be tempted to get the linear type whilst a lot cheaper as they render Autofocus and some metering systems will not work accurately

Danny O'Byrne  http://www.digitalartzone.co.uk/

"All the technique in the world doesn't compensate for the inability to notice" Eliott Erwitt