Ask HN: Promoting a startup. Tips?

I've had 2 startups so far- both with limited users. I seem to be having a huge problem generating traffic (yes, it might be due to the fact that the site itself wasn't interesting enough).

The question is: How do you generate traffic for your new startup?

I want to hear about some ways that you experts have successfully used to promote your site. The most common ones already explained are SEO, paid advertising, twitter/ facebook. Any special methods?