FIRST Sentence and Paragraph
Various SEO books and experts all agree on the importance of including keywords in the first sentence and paragraph of your article. I concur with that advice.
For example, if your key phrase is “global warming” here is a suggested first paragraph:
“Global warming is more than a speculation right now. Scientists, through careful study of fossil evidence and sedimentation analysis, have proven that global warming is a reality. That calls for the implementation of careful policies to reduce the emission of greenhouse gasses and reduce the consumption of carbon-based fossil fuels.”
Most SEO experts would recommend including your keyword(s) at least for once in your last paragraph as well. I would concur with that as well.
Some SEO experts also claim that roughly 5-7 % of your word count should consist of your keyword. For example, if you write a 300 word article, you should roughly include your keyword 15-20 times, title included. The idea if not overdoing it in order not to get flagged by the search engines for “keyword stuffing.” However, if you include only 3 or 4 keywords in a 300-word article, that might be too little as well. So use your common sense and judgment in coming up with the right mix.
Some sites like SiteSell have their built-in keyword frequency checking program that gives you a feedback about your keyword density as well as if keywords are missing from your titles, etc.