I've written a lot about keywords both as a ghostwriter and under my own name. I just recently published this keyword article on Suite101. I'm starting to wonder, however, how important keywords really are.
Most of the keyword information out there is just anecdotal- search engines do not tell you to have any word repeated over and over. They do not specify keyword densities. Yet, everyone believes that if they just have the right keyword density, they will rank better. Based on my own anecdotal evidence, I'm starting to wonder if that's true. I suspect that it may once have been true but that search engines got hip to the keyword density race and scraped it in favor of content that sounds more natural.
In my own experience, and I do Internet research for hours each day for various writing projects, keyword density doesn't do a heck of a lot. I pull up articles every single day that fleetingly mention a topic once and yet are featured on the front page of results. I've also seen that I rank in the top one or two results for several search terms, and I have never put any term in this blog with an eye toward keywords or keyword density. Some of the most obnoxious blogs in the world are ones that peg a niche and flood it with keywords, or worse- awkward keyword phrases.
What does look like a better way to get a ranking is to continually add to a page or a website. The top-ranked pages are almost always ones that are new or that are added onto frequently. If you really want to get a better ranking, I highly recommend taking the time to add onto your site every few days or even more often. And don't worry about keyword density- there's no proof that search engines do.