Writing content for search engines and your visitors
Its important to remember when writing content for your website that you are writing for humans and not Google. Far to many websites stuff their content full of keywords in the hope of google indexing them higher. The reality is that Google and its spiders are not stupid. There was a time when the method of stuffing keywords on a page was successful, times have changed, therefore your techniques should also. Back in the early years of search engines, one could easily manipulate a page’s ranking in Google with keyword stuffing.
In the not to distant past websites could rank on a large variety of keywords by simply cluttering them onto a page, most people strangely aren't looking to find a web page that just reads the same words repeated 500 times (strange I know). Google and other search engines wised up and began filtering out keyword-stuffed pages, because these pages were largely devoid of useful content.
So how do you use keywords responsibly?
Some say there is no specific number. Others say there is: 2-5% keyword density is considered safe by most.
Instead of using the same keyword a hundred times, try using some long-tail keyword variations in your content. There are plenty of handy tools that can help you generate long-tail variations, take a look at PPC Keyword Generator.
Another way is to implement synonyms (various words with the same meaning). Search engines recognize that words like “bow” are homonyms; they are words that have multiple meanings. Google prides itself on relevancy, someone searching for a Weapon “bow” vs. a Ships “bow.” For this reason, there's a database index of Google synonyms to help it differentiate between word meanings. Google knows that if a site is talking about “Arrows” and “Bows,” they are probably talking about weapons and not boats.
Take a look here for some homonyms.