You don't have to obsess about keyword density, you can rank for a keyword with just 1 exact match keyword in your article as much as you can rank for some keywords without any backlinks. You need to have that keyword within the article or metadata (title, description, tags etc.).
A typical example SEOs use to prove that keyword density is not a ranking factor is by using the keyword "Miserable Failure". There's an about.com page that is ranking for this keyword in the SERPs but the page doesn't have any text on it, there's only an image on the page and nothing else but it's ranking in the top 10.
The keyword is only in the title, description and tags (meta keywords). That's how SEOs prove that keyword density doesn't matter.
So my advise:
- Have the keyword in the title
- Add it in the description and tags. People may argue that google doesn't look into these but the "miserable failure" example shows us otherwise. They'll tell you there's a lot of factors blah blah... just put it there, it doesn't hurt anyone if you have it there anyway
- Use it in the H1 or H2 tags within your article
- Have it in the first paragraph of your article
- Have quality content that reads naturally
Obviously there are other factors that may affect rankings besides keyword density but I won't tell you that because I just want to answer your question and stay on-topic. I know some people will come and tell you there are other factors blah blah blah... but you already know that and that's not what you're asking.
So don't obsess about it, use it in the title, description, tags and within the article as well. How many times you use it doesn't matter but don't spam, too much of anything is never good.
