Don't discount all directories. A good link from Open Directory can work wonders. Plus, don't pay attention to the Google Toolbar Page Rank indicator. I've had sites go up and down in that yet the SERP's for them remain steady.
You want imporve the page rank no, start the directory submission, article submission, blog posting, social book marking, forum signature and press releases, free classifieds and also try for link exchanges.
I think directory submission is nice way but for good PR you need some more things to do such as social bookmarking, blog posting, article posting etc. it will help you for back links as well as popularity.
Thanks for the replies people. I've only done a little bit of directory submission and forum/blog posting and I noticed that it does help. I know some people says that PR doesn't matter. I know. But it for my personal satisfaction.