You should use the heading tag to organize your content.
The h1 tag is the main point (usually a keyword). You can only have one h1 tag for proper seo, and it does count quite a bit today.
The h2 tag should be used to bring in a related keyword into the page, or expand on the subject matter of the title and h1 tag.
If you are in the USA, think of the research project outline you were taught in school.
If your research project was about "Whales of the Atlantic" (BTW, I don't know jack about whales, I am just making this up as I go along), your outline would look like this.
Whales of the Atlantic.
1. Plankton Feeders
2. Krill Feeders
3. Live feeders
In this case the title of the project is the equivalent of your h1 tag, and the numeric divisions under the title are the h2's.
<h1>Whales of the Atlantic,<h1>
<h2>Plankton Feeders</h2>
<h2>Krill Feeders</h2>
<h2>Live feeders</h2>
In the same manner, the h3's should be a related expansion of the H2, just as if it were an extension of the outline:
Whales of the Atlantic.
1. Plankton Feeders
A. Baleen Whales
2. Krill Feeders
A. Right Whales
3. Live feeders
A. Orcas
Becomes:
Whales of the Atlantic.
<h2>Plankton Feeders</h2>
<h3>Baleen Whales</h3>
<h2>Krill Feeders</h2>
<h3>Right Whales</h3>
<h2>Live feeders</h2>
<h3>Orcas</h3>
You can go 6 levels deep. Each level will produce a smaller font, and the weight given the words contained in these headings will be given extra importance in the algorithm. You are telling the search engines that this information is more important than other information on the page. The higher the h tag, the more influence the words are given.
For an IM site,this structure works out like this.
<h1>How to make money online</h1>
<h2>Affiliate sales</h2>
<h3>Clickbank</h3>
<h4>Using a landing page</h4>
<h4>Direct linking</h4>
<h3>Amazon
<h4>Using a amazon store</h4>
<h4>Using a product review site</h4>
<h2>Cost per action
and so on.
If you craft your content carefully, you can legally keyword stuff the hell out of the page.
Two bonus tips.
When you write this way, the heading tags are surrounded by related terms that appear only in this section. This is good.
I always use an H3 tag for a last line on the webpage. Not in the footer. On small business pages, this H3 tag contains the NAP of the business in schema markup (a footer is fine for this, it geo locks the site). In other sites, it contains a copyright statement along the lines of
"copyright 2013 | keyword here | all rights reserved"
So one page might be
"copyright 2013 | Affiliate sites using Amazon | all rights reserved"
and the next page might be:
"copyright 2013 | Affiliate sites with ClickBank | all rights reserved"
Google doesn't seem to mind, probably because it is at the bottom of the page.