Don't omit alt text for images especially images inside the text (AVOID)
Images inline with your text can dress up your Web page, but if you leave off the alternative text (alt text) search engines won't pick up the content relevance. Also keep in mind that the heavier your page is with images, the less likely that search engines will rank it highly. Text is what gets ranked in most search engines, and alt text is a poor alternative.
Don't use images instead of text links (AVOID)
Search engine optimization is all about text, and if you use images instead of text, even if you have good alt text, search engines will have a harder time ranking your site. This is especially true for navigation. Search engine spiders crawl through your site by following links, and links on images can be more difficult for them to follow or rank than text links. Using images instead of text makes your pages slower for your customers too. You're better off styling your text with CSS, than using images.
Don't misspell your keywords in your content (AVOID)
It can be very tempting to try to optimize your site for misspellings. And while it won't hurt your site in the search engine rankings - especially if you decide to use the misspelled version as your keyword phrase to optimize on. It will hurt your credibility with your customers. For every one person who misspells the word, there are at least two to three who know the correct spelling. And if they end up on your page for some reason, they will just think you are unprofessional. Plus, many browsers and search engines have spell checkers built into the forms, so the popularity of misspellings will continue to lessen as time goes on.
Don't try to optimize for more than 2-3 keywords and phrases (AVOID)
This is called keyword dillution. If you have too many topics on a given page, it will be hard for both search engines and your customers to determine what you're talking about. If you have a lot to say on several topics, it's better to write multiple short pages on each topic, than to try to cram them all into one long page.
Don't use your keyword phrase too much (AVOID)
Keyword stuffing is the practice of repeating your keywords or keyword phrases over and over in a page until there is nearly no other text than the keyword phrase. Check your keyword density to determine if you have used it too much. 10% or higher is too much.
If you are too blatant about stuffing keywords, you could get your site banned from search engines.
Don't rely on links from domains on the same IP (AVOID)
While Google doesn't discriminate against domains that have the same IP (for example, domains that use virtual hosts), other search engines may. So it's best to avoid trying to increase your inbound links with links from other domains that you own. The same is true for domains hosted on the same hosting provider (coming from the same C-level IP address). Google doesn't penalize sites like this, but other engines might.
This is another situation where if it becomes apparent that you're doing it, you could get all your sites banned from search engines.
Don't have more than 10 words in your URL (AVOID)
While you want to have keywords in your URL (and domain if possible), longer URLs tend to look more spammy to both customers and search engines. However, this isn't a serious issue, and if you need to have 11 or 15 words in your URL, it shouldn't be a problem, as long as you aren't doing it all the time.
Don't use URL parameters if you can avoid it (AVOID)
Parameters on URLs make them long and hard for anyone to read. And search engines can get confused by them, especially if the parameters are meant to hold customer information, and not indicate a separate Web page. Also, as I mentioned elsewhere, search engines don't always rank dynamic pages as high as static pages, and most dynamic pages use parameters on the URL to indicate the correct page. If you must use parameters, you might want to consider doing a URL rewrite to static URLs, at least for your most important pages.
Don't use dynamic URLs (AVOID)
In general, spiders tend to prefer static URLs to dynamic ones. It is possible to rank high with a dynamic URL, but it's easier if you redirect dynamic URLs to shorter, static URLs.