Avoid "keyword stuffing"—forcing keywords into content. Focus on high-quality content instead of optimal keyword density. While keyword density is generally considered to be most advantageous around 4%, do what works best for your site.
The same holds true for SEM. While keywords and SEO are important, it's user experience that is key to generating conversions. For SEM landing pages, be sure content is relevant and the conversion action is clear. If a user can't discern the action you wish them to take, they'll likely hit the browser "back" button.
The actual design and layout of a site may be beautiful, but users can immediately sense if the site contains what they want. Have pertinent content immediately available; avoid forcing users to dig to find it. Also, be sure not to overload your page with content as it can lead to some of the same problems as keyword-stuffing.
When writing content, keep it clear and concise. Use relevant terms where they sound appropriate, and avoid overloading a page with text. Ensure content fonts are legible and avoid overusing italics and bold.
Avoid repetitive content. Be sure all content is page-specific based on targeted keywords. The targeted keywords should match the keywords in the searcher's query or be strongly relevant. Home page keywords should be broader and pertain to the company's overall products and services. Furthermore, be sure that as a user travels deeper within a site, the content on the page matches the keywords and is unique to that page.
Worry less about how your site compares via search engine algorithms and more about sensible content and calls to action. You may have relevant content according to the search engine algorithm, but that's no guarantee users will feel the same way.
Ashley Bailes is a client strategist at MoreVisibility, Boca Raton, Fla. She can be reached at email@example.com.