By far, my most successful marketing/advertising campaign is through
SEO.
Although I don't consider myself an expert, I was able to drive a lot of traffic by focusing on small, but high demand keywords.
What I do is go to Wordtracker and research related keywords. Then I run a KEI analysis and find the "low-hanging fruit" of the Internet. Basically, these are the words that have a medium demand, with very little supply.
Finally, I follow basic
SEO tricks and create a unique page for each keyword phrase.
This is more of long term strategy, but it is well worth it when 3-4 months down the road, you receive a ton of traffic.