As Steve mentioned, one of the key things is search engines and, although they can cost a lot of money, there are a number of things you can do to get the most out of them at very little cost.
The biggest and most popular search engine is Google and one of the most important factors they consider is 'link popularity'. Generally speaking the more webistes that link to yours, the more important they consider your site to be.
Every member here has the opportunity to list their website in the profile section once they have logged in. This also ensures that your site doesnt get missed when they 'crawl' the web indexing new sites.
It helps and it's free