Far too many websites are completely over optimised for various keywords. This tactic used to work back in the day when Google wasn’t quite able to combat spam, but nowadays it’s a hindrance. Of course, there are no clear rules as to what constitutes as ‘over optimisation’, but if you have a keyword density of 30 or more for one particular phrase, such as ‘T-shirts for sale’; Google may mark you down for producing poor quality content. Google wants websites to be human friendly, so that means not having too much repetition for certain keywords. Try to pick similar words and create some sense of variety, otherwise you may bore your visitors, or worse, drop down in the Google rankings. There are many tools on the market that can help you check your keyword densities fast, and many of these come as free plugins for your browser.