Don't link a lot to external sites (LOW PRIORITY)
Linking to sites not on your site is a good idea, but don't fill up your pages with them. At best, you will dilute the effectiveness of your page in the search engines, and at worst your page will look like a list of links and get slightly penalized by search engines. Also, when you have lots of external links, you have more to check on a regular basis, to make sure that those pages don't go bad or turn into "bad neighborhoods".
Register a separate domain instead of a sub-domain (LOW PRIORITY)
Subdomains are a nice way to create new websites without needing to register a new domain. This site is a subdomain of About.com - webdesign.about.com. But subdomains are not as recognized by search engines (or customers for that matter) as separate sites. For example, most people who link to my site link to it with a title of "About.com." But if you were to go to www.about.com, you'd get a very different impression of my site than the true URL of webdesign.about.com. The other problem with subdomains is that most people think that URLs should start with "www". Sometimes www.subdomain.domain.com will work, but sometimes it won't.
If you can, you should move all sites that are on a subdomain onto a real domain name of their own.
Register a .com domain over a .biz or .us domain (LOW PRIORITY)
Trying to find a good domain name can be challenging, especially on the .com top-level domain (TLD). But finding a good .com domain will rank higher than a similar domain on the .biz or .us TLDs. And if you can get a .edu domain (because you're a school or university) your site will have more credibility instantly. Some SEO services feel that a .org TLD is better than a .com, but they aren't any more difficult (in general) to get than a .com domain, and while search engines might give them some priority now, they will probably lessen that as .org domains become more common.
Use hyphens to separate words in domains (LOW PRIORITY)
When you're putting keywords in your domain and URLs, you should consider separating them with hyphens (-) rather than mashing them all together or using underscores (_). Search engine spiders can't tell where a word ends and begins without cues like hyphens, and most computers recognize hyphens as the end of a word, but see underscores as part of the word.
Use hyphens or underscores to separate words in URLs (LOW PRIORITY)
Just like your domains, you should separate words in your URLs with hyphens (-) or underscores (_). Hyphens are better, but outside of the domain, underscores can work. Hyphens work better because many search engine spiders recognize hyphens as the end of a word, but see underscores as part of the word. Also, underscores can be seen as a space by your customers (because the underline of the link and the underscore merge together), and they will then get frustrated if they try to type the URL with a space and can't get to the page.
Write short pages (LOW PRIORITY)
The shorter your page is, the fewer times you need to repeat your keyword phrase and keep the density just right. Plus, short pages load more quickly, and so your readers will appreciate it. Keep pages under 30KB in size. Split long pages into multiple pages and optimize each page.
Include text transcripts of podcasts and sound files (LOW PRIORITY)
Like images and Flash, search engines can't index the content of sound files including podcasts. By including a transcript of your sound files and podcasts, you give search engines more text to index.
Don't host your site with a host that allows spammers (AVOID)
This means any type of spammers, but especially search engine spammers. If you don't know what your host's policy is towards spammers, find out. There should be something in their terms and conditions about malicious activity. If your IP is blacklisted, you'll be blacklisted right along with it, even if your site is completely innocent.
Don't host your site with a host that is down a lot (AVOID)
While search engines won't deliberately discriminate against a site that is down, if they can't get to your URL because it's down, they can't index it. And if your site is down several times when the spider tries to access it, it could be flagged as gone, and then the spider won't come at all. Find out from your hosting provider what their uptime rates are and what they guarantee. Less than 97-98% uptime is bad.