Your header tags can be great tools in aiding search engines in their quest to determine what your web site is about. I have seen many sites that completely ignore the placement, structure, content and use of these vital tags to their peril. Using these these tags correctly and effectively can help your on-site SEO efforts dramatically.
The header tags that carry the most weight are the h1, h2 and h3 tags. The h1 carries the most weight and influence with the search engines. As you might suspect the h2 tag comes in second and the h3 in last with as far as influencing the search engines. Using more than these three for on-site SEO is not an effective strategy because the weight the remaining header tags carry after the h3 tag is negligible.
The placement of these three header tags is important. The h1 should always be above the h2 and the h2 above the h3. With the h1 tag I have found success with placing it closer the the upper left hand corner. Search engines studies have determined that this is one of the first places the eye travel naturally. In my own experience I have seen the weight of the h1 tag coupled with the upper left placement drop listings for websites on the SERPS (Search Engine Results Pages) by more than a page!
The content of the tags bears a heavy influence on search results. First and foremost you want text not images for content. You also want that text to be short and concise. A standard rule of SEO is to optimize each page for not more 1-3 keywords and this should be reflected in the header tags.
Using your header tags correctly with SEO in mind you can ensure your site is getting the most from its on-site SEO. Together with other SEO best practices this technique can get you the SERPS listings and traffic your site needs to prosper.