A good question.
I have found that by keeping the keyword density around 1.5% to 2.5% that I have not had any problems - as long as my writing flows well. Trying to make things sound good just to make sure that you have the kw density down seems to hurt. Lesson: Write for human beings.
Overall my article marketing strategies have not changed that much and I still have #1 positions for some key phrases with medium or above levels of competition.
The key, in my opinion is to always write content that is relevant. I also read an article on anchor text and how partial match anchor text seems to be pulling ahead of the exact match anchors when you are promoting your blog posts or articles and building backlinks.