It makes a lot of sense to advertise your site using pay-per-click ads on sites like Google.
But, if you can get high placement with the "natural search results" on Google and others, that's even better.
This has been my current strategy. I've been using a free tool called Hittail from Connors Communication, a PR firm.
It allows me to see what people are searching for when they come to my site, then it does some analysis on those search phrases and categorizes them as to how important they are.
For instance, right now I'm seeing that the search phrase "Daryl Kulak" comes up a lot for people coming to my blog and Websites, but it is not important that I blog about "Daryl Kulak" because I'm already high in the rankings for that phrase (imagine that).
But, there are phrases like "holistic marketing" and "health savings account downside" that I should be blogging about, because those are terms where people came to my site, but I am quite low in the rankings on those ones.
I am really enjoying using Hittail. I'm glad it is a free service, and I'm thankful to Connors for providing it. Also, I really like Mike Levin's blog about Hittail, it's very informative and he updates it often. Hittail is a bit hard to understand, so I suggest you subscribe to his blog and let him teach you about it each day, a little at a time.
When you sign up, you will have to embed a code snippet in your blog or Website, so make sure you have the capability of doing that, or have your Webmaster take care of it.