Search engine optimization (SEO) can be divided into two categories: on-page and off-page. On-page optimizations are minor modifications of page elements to emphasize important keywords. This means adding or modifying header tags, meta tags, image alt tags, page title and other relevant parts of the page. Another method to emphasize important keywords is to change keyword frequency. Unfortunately, many such SEO modifications can make pages to look unnatural. To avoid this unwanted effect you should follow few webmaster guidelines.
The first rule is to make a website for users and to optimize it for search engines. You should not focus on Google, Yahoo, Bing or any other search engine. Focus on visitors that will come to your site looking for some information. Make pages easy to read and easy to navigate. You should never change the text to increase keyword frequency just to be more attractive to search engines. You should remember that the purpose of the website is to attract human visitors in first place. The only aim of SEO should be to clearly label the content of the page. Search engines see our website differently than human visitors. Therefore, there are few page elements that can be used to emphasize important text or keywords.
The best approach to create good text that will read naturally is to write it as you would write for a book, newspaper or any other media. Use you own words and do not repeat target keywords unless it is in the context of the page content. Natural text will still contain enough target words which will be spotted by search engines. Excessive repeating of particular words or phrases make pages hard to read and thus less attractive.
With a very simple keyword frequency check you can ensure that the text contains no excessive repeating of any word. There are many web tools that can check word frequency and there is even a plugin for Firefox that can analyze page content from various aspects including keyword density. If you find that some word has a very high frequency then try to modify the text accordingly. In most cases you will be able to create quality text without excessive word repeating.
There is no rule for “normal” keyword frequency. All you have to do is to make the text to look and read natural. If you have used header tags and the page has all relevant meta tags then the basic on-page SEO is already done.