Choosing a poor domain name.
Not surprisingly, URLs containing clear keywords generally perform better than those that appear random or are excessively long. And in addition to getting you higher placement in search results, having its own domain name gives your site added credibility. You'll want to make sure the name you choose logically pertains to the subject matter of your site, isn't too long to remember, and isn't easily misspelled. Use keywords that a crawler will understand, and try to avoid numerals or abbreviations.

Not using keyword-rich titles for your pages.
The [TITLE]; element that appears in your page headers is often used by search engines as the text for their link to your Web site. (As an example, the title of http://www.amazon.com/gp/browse.html/?node=11091801 is "Amazon.com: Musical Instruments.") When you just use "Home Page," your company name, or something similarly uninformative, you are missing an opportunity to drive traffic to your site, since search engines weight [TITLE] elements heavily when determining the relevance of a page to a user's search. Therefore, try to make your titles easy to understand and rich in the keywords that your customers will be searching for.

Improper use of META elements in your site.
Make use of description and keyword properties in your headers' META elements. META name="description" content="[a brief description of your site]" is often used by search engines to determine what your site is about. META name="keywords" content="[a list of relevant keywords]" is used less frequently by search engines, but can also help boost your site's relevance in their eyes. In writing a description and choosing keywords, think about how customers will be looking for information on your site, and choose specific terms that will attract traffic. You may want to use research tools like Wordtracker to help you in this. Avoid using the same set of keywords on every page of your site, however--they should be tailored to each page's specific content.