There are a ton of factors that go into it, including appropriate keyword density (that doesn't mean spamming the keyword, which actually has a negative influence), and the location of your main text. For instance, let's say you have a 3-column website. It's much easier to code that in such a way that the middle column is at the bottom of the code (which is where most people would put the main text). However, if you spend the extra time and effort to put the main text area at the top of the code, it has a greater influence on the
SEO.