http://guides.seomoz.org/chapter-4-basics-of-search-engine-friendly-design-and-development

That said, keyword usage and targeting are only a small part of the search engines’ ranking algorithms, and we can still leverage some effective “best practices” for keyword usage to help make pages that are very close to “optimized.” Here at SEOmoz, we engage in a lot of testing and get to see a huge number of search results and shifts based on keyword usage tactics. When working with one of your own sites, this is the process we recommend:

• Use the keyword in the title tag at least once, and possibly twice (or as a variation) if it makes sense and sounds good (this is subjective, but necessary). Try to keep the keyword as close to the beginning of the title tag as possible. More detail on title tags follows later in this section. 
• Once in the H1 header tag of the page.
• At least 3X in the body copy on the page (sometimes a few more times if there’s a lot of text content). You may find additional value in adding the keyword more than 3X, but in our experience, adding more instances of a term or phrase tends to have little to no impact on rankings. 
• At least once in bold. You can use either the or tag, as search engines consider them equivalent.
• At least once in the alt attribute of an image on the page. This not only helps with web search, but also image search, which can occasionally bring valuable traffic. 
• Once in the URL. Additional rules for URLs and keywords are discussed later on in this section. 
• At least once (sometimes 2X when it makes sense) in the meta description tag. Note that the meta description tag does NOT get used by the engines for rankings, but rather helps to attract clicks by searchers from the results page (as it is the “snippet” of text used by the search engines). 
• Generally not in link anchor text on the page itself that points to other pages on your site or different domains (this is a bit complex – see this blog post for details).