I'm not sure why and if it helps, but one of my directory sites (
www.pembdirectory.co.uk ) has jumped from PR3 > PR6 in just a few months. As far as I am aware, there are less than 100 backlinks to this domain.
I do constantly update this site and some of the featured links go to good
PR sites. Plus of course it features in my signiture below (which I'm sure helps)..