Avoiding Search Engine No-No's

Invisible Text - This technique was last fashionable circa 1996. Search engines detect it easily and will likely penalize or remove your pages from their index for using it. This technique is especially abusive if the invisible text is unrelated to the content of the page it's on.
Frames - Frames are not spam per se, but search engines have trouble with them. When they encounter a frame, they either stop because they can't navigate beyond it, or they index the other pages in the site in such a way that visitors land on them directly, outside the frame in which they designed to be viewed. If your site uses frames, your only real alternative is to redesign it without them.
Duplicate Content - If you set up an affiliate site with the exact same content as other sites, significant directories such as DMOZ won't accept you. If you plagiarize other sites, you may be sued. In either case, the search engines may detect your use of duplicate content and penalize you.
Flash and Overuse of Multimedia Content - Your home page stands a better change of getting a high ranking for your two or three most important key word phrases than any other page in your site. If a Flash intro page is the first page in your site, you throw away that advantage. Search engines can't index Flash content. Often, Flash intro pages are designed such that search engines can't navigate beyond them. Also, sites that overuse graphics, Flash, music, animation, etc. usually don't offer much text for search engines to read. They also aggravate users with slow connections. So go easy on this stuff. If you use some of it, try to get a couple of hundred words of actual text on your home page as well, and use ALT-tags to describe images for text browsers and search engines.
Redirects - A website using redirects may be indexed but probably won't rank well because the engines won't be able to see the relationships between its web pages. This is because unlike direct text links, redirect code interferes with their ability to navigate. If used to deceive visitors (e.g., redirect them from a page designed solely for the search engines to a page you really want them to see), redirects will get your site penalized.
Dynamic URLs - Search engines have problems indexing dynamic shopping and e-commerce sites that use parameters and session ID's in their URLs. See my article on "Ensuring that Dynamic Sites Get Indexed" for a thorough discussion of this issue.
Incorrect Robots.txt file and/or No Index Tag - Sometimes webmasters will use these to intentionally block search engines while their site is under construction, or to exclude them from certain areas of a site. All too often, they forget to remove them once the site is built, or write their robots.txt file incorrectly such that it bars search engine spiders. Make sure you don't have a robots.txt file in the root of your website containing "User-agent: * disallow /" or a Meta-tag that reads <META NAME="ROBOTS" CONTENT="NOINDEX, NOFOLLOW">
If you are using these tags during beta testing, don't forget to remove them when you go live.
Doorway Pages - Several years ago, many SEO professionals used software to auto-generate hundreds, even thousands of doorway pages - pages optimized for a particular key word/search engine combination. A site promoting 100 key word phrases might have included 1500 doorway pages - one for each key word times about 15 search engines. Needless to say, search engines hated this tactic. It clogged their indexes with pages designed merely to lure visitors, not provide useful content. They have long since learned how to spot these pages and they'll penalize your entire website for using them. You may need to start from scratch with a new domain name to recover from such a penalty. In spite of this, some SEO firms still build doorway pages for clients. If you have a lot of relevant key word phrases, you have two legitimate alternatives: build a page containing unique content for each of these phrases (or at most, two or three of them) or bid on them in the pay-per-clicks. If you're selling many products, include all available text about each of your products on it's own page and optimize that page for that product. If you do everything else right, that page stands a good chance of ranking well for searches on that product. Until it does, pay-per-click ads can lead visitors directly to it.
Identical Titles and Meta-Tags - Use unique titles and meta tags for every page in your site. If you use the same tags on every page, the search engines may think that all of your content is exactly the same. On each page:
Title tags should be less than 80 characters and contain the two or three key word phrases most relevant to that page.
Key word meta tags should contain those same phrases. They can include additional phrases (after the most important ones) but shouldn't contain phrases not found in the body text of the page.
Description tags should provide a call to action and should accurately describe what the page is about.
Link Farms - Avoid services that offer to link thousands of other websites to yours. Other sites can't hurt your site by linking to it. If they could, unscrupulous competitors could harm other sites by linking to them. But unless these sites are relevant to yours and themselves well placed in the search engines, such links won't help you. On the other hand, linking your site to penalized sites can hurt you, so be careful. The best way to build links is to make your site a valuable resource such that other sites will want to link to it. Search engines view such sites as "authorities." This may take a while, so you way want to pursue reciprocal links, but choose your link partners carefully. Search engines view sites linking to related, high-quality sites as "hubs". Make your link directory a valuable resource of related sites that will interest your site visitors. A catch-all directory of unrelated, low quality sites will drain your PageRank and may get you penalized.