Content, Content, Content
Let's be honest this is the key element. Hopefully, it is what your site is about. There is a lot of talk about making sure that all your keywords are in the body of the pages of your site but you'll probably find that if you are writing about what you do and offer they are already in there.
However, it doesn't harm taking a close look at your pages from time to time to make sure that you have covered all your bases. Here are a few pointers to helping the search engines index your page and improve your rankings.
It is worthwhile making sure that your keywords link to appropriate places on your site, so it is better to use something like "read our guide to seo" rather than something like "to read our guide to seo, click here". Click here does mean a lot and search engine do put some weight on linked text.
Highlighting keywords using <b>bold</b>, <em>emphasis</em> and heading tags <h1><h2>... etc, can also help the search engines identify them.
Make sure that the content is unique - don't repeat what is on another page, make sure that at least 70% of the content is not repeated elsewhere.
Not too long. This can get you penalised on Google and it is not good for your audience either. If it takes more than a second to scroll to the bottom, it's too long.
All text should be visible. Do not hide text on the page. Google doesn't like it.
It has to be relevant, if your site is about writing books and your page is about getting them published, then the content must be about getting them published - sounds obvious but it is easy to wander off the beaten track.
So how do the search engines decide on relevancy?
They look at your Title Tag, Meta Tags and the URL of your page fro keywords and then look to see how many times these appear in the content of your page. Too many and you get penalised, too few and it doesn't look relevant. So what's your keyword density? Use this keyword density tool at SEO Tools to check your pages.