how to calculate viral titer? - (Jun/30/2010 )

Hi, I'm looking to calculate viral titers (retro) that I'm getting from Phoenix cells. I'm not sure what's the best route, I want something along the lines of a number of viral particles/vol of viral supernatant as a result. I've been told this can be done but nobody seems to know quite how Help would be appreciated.

if you want to know the exact number of particles you need to do Plaque Assay.