From my own experience, I can name the main ways to promote your business:
1. embrase social media
2. start a blog
3. put up multimedia on YouTube and Flickr
4. SEO your site
5. join a relevant online community and contribute
And I agree with the statements above, especially with one about money. Almost all ways of promotion require investment. A lot of business owners take out loans. And I'm no exception. I remember how nervous I was. But addressing a target cash now customer service here and getting money there was a wise decision as it was very helpful for my startup.