To add to what Gary has mentioned, you can write articles about your topic and submit them to article directories. You can create a blog at one or more popular blog sites such as blogger.com or wordpress.com and blog about topics relating to your site and then place links as appropriate back to your site. You can also create lenses on Squidoo or hubs at HubPages.com relating to your topic and add links to your site from those places as well.
On top of this forum, you can find forums relating to topics that you cover on your site and place signatures there as well. If you come across a blog post from other sites that are related to what your site is about, you can comment on the post and usually have the ability to post a link back to your site as well.
Look at other sites similar to yours and ask for links from those sites to yours. This will help as well.
Ted