Here is a great article posted by Mandy Barrington on Sitepoint.com on February 13, 2012

If you want traffic from search engines, you know that you need to optimize your page for your target keywords. But where on the page should you place your keywords?

Imagine your target keywords are seeds: will you throw them in the air and hope they grow wherever they fall, or will you place them precisely where they’ll get the most sunlight (and capture the most attention)?

When you look at the big picture, popular search engines like Google have been so successful because they are able to determine the most relevant websites for the keywords queried, while penalizing sites using spammy tactics. This has led to a more satisfactory search experience for users, and a more challenging experience for web marketers and developers. Read full article