Recommendations for promoting a new web app?

I've been working on a web app for a couple weeks. I should be able to launch it in a few weeks. So if anyone reading HN has had success at building buzz, what'd you do? I know the basics: post on HN, share on Twitter, submit to Digg, send to Techcrunch, etc. Going to get some of my buddies to promote it as much as they can, but none of them are big web celebrities. I feel certain that if you build a project that is simple, works very well for its purpose, and makes it easier for the user to do a task, then it has a better chance at success.

I made a Firefox extension about 3 years ago called "Cacheout!" that was up to about 10,000 regular users before I had to kill the project when my hosting situation died. It had gotten to the front page of Digg and had been shared around a few blogs. I considered it a minor success for me at the time.

What should I do to get ready to promote this new project?