Make an effort to include keyphrase(s) in the headlines and sub-heads if at all possible. Adding keyphrases about once or twice per paragraph is a good goal. I never count words or run keyword density percentages. Focus on writing in natural language. Yes, you want to incorporate keywords, but not to the point that you ruin your copy. It should always sound natural.
Read your copy out loud. If it sounds stupid or redundant to you, it will sound stupid and redundant to your site visitor. If it makes sense to do so, I try to include keyphrase(s) in bold, italic, bulleted lists, or in other text that is specially formatted. If you wouldn’t bold or italicize the words or phrases to emphasize them to your visitors, however, don’t make a special exception for the engines.
Bottom line? Don’t sacrifice the quality and conversion power of your copy to chase search engine rabbits. In the end, it won’t be worth it.