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.
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 dilution. 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.
Don't use session IDs (AVOID)
Like dynamic URLs, search engines don't tend to like URLs with session IDs on them. In fact, session IDs seem to cause even more problems with search engine spiders than plain dynamic URLs. The problem is that every time the spider comes to a site with session IDs it can index that site as a completely new URL - even though the content is identical. This can lead to the search engine thinking you are trying to spam them with identical content, and could even get your site banned if it got bad enough. Google guidelines now state that id= URL parameters are okay, but that doesn't mean that other search engines won't choke on them.
Don't rely on AdWords to boost your rankings (AVOID)
AdWords is a way to advertise your sites on Google. While you can pay to get high rankings in advertising venues, having an AdWords account won't help your rankings in natural (non-paid) search, even in Google. It won't hurt your rankings either. You can use AdWords to get more clicks to your website, but they will appear only in paid search locations, not in the natural search.
Try to get your site off link farms (AVOID)
You should never link to a link farm. And while search engines state that they don't discriminate against sites that are linked to from link farms, it's a good idea to try to keep your site off of them, if only to avoid contamination by association.
Don't link to link farms (AVOID)
Google refers to spamming sites as "bad neighborhoods" and if you link to them, you will end up with a lower PageRank. If you suspect that a site you want to link to is a "bad neighborhood", check their PageRank and see if they commit any obvious SEO no nos. If they do, or you think they might, then you shouldn't link to them.
Don't link to and from the same site repeatedly (AVOID)
This is also called link spamming. At best, search engines will look at the links you have on your page, and only count the first one or two towards optimization. And at worst, your site might appear to be a spammer, even if you're not linking to a "bad neighborhood" or are in a cross-linking scheme. You want to avoid looking like you are paying for links.
Don't get into link circles (cross-linking) (AVOID)
When several sites have links set up in a circular (or more complex) pattern (site A links to site B links to site C links to site A), it can look like you're paying for links. Don't assume that because your average customer won't notice the pattern, the search engine won't either. Since search engines give some priority for links, they want to reward "honest" links, or links that are not paid for. If it looks like you might have paid for the links (even if you haven't) your ranking could be penalized slightly.
Don't have broken links on your site (AVOID)
Broken links make your site look bad, and they imply that you don't manage your site very much. Search engines want to have only the highest quality results, so they may penalize sites with lots of broken links. Use a link checker periodically to make sure that your links are still valid.
Don't use the meta refresh tag to redirect users (AVOID)
It can be very tempting to set up redirects on your site with the meta refresh tag, but this can be a bad idea. Many spammers use them to try and fool search engines into thinking that a page is about one thing, and then refreshing to something completely different. Meta refresh also doesn't give information to the search engine about why the redirect is occurring. It's much better to set up a permanent HTTP 301 redirect when you need to redirect your customers to a new URL.
Don't use 302 redirects (AVOID)
A HTTP 302 server redirect is supposed to be used when a page is only temporarily moved from one location to another on a server. Spammers use 302 redirects because that gives them many more URLs to the same final page and thus many more ways to get to that page.
The only time you should use a 302 redirect is if you have ugly URLs with lots of parameters on them. The 302 redirect tells the search engine that this is not a permanent redirect, but rather an alternate URL. For all other redirects you should use a 301 redirect instead.
Don't make constant minor changes to content (AVOID)
While you want search engines to see that you update your content, making minor changes (like correcting spelling errors, or changing 10 or 20 characters) implies that you're just trying to get the updated date changed. This looks like you're trying to fool the search engines into thinking that you update your pages more than you actually do.
Do spend time updating your pages, but make the updates substantive.
Don't separate content artificially (AVOID)
If you really must display alternate content based on some artificial measure, create separate Web pages for each, rather than using the same URL for all the content. Or, keep the content that is different as minimal as possible, don't build an entire new site for each IP or browser type.
Don't violate copyright or other laws (AVOID)
Most search engines have terms of service that ban sites that break the law. Copyright infringement is the easiest way to break the law on the Web. Don't assume that because something was posted to the Web it is legal for you to reprint it, get permission or link to the article instead of copying it. Search engines will ban your site if you regularly steal content or break other laws.
Don't duplicate content on your site (AVOID)
One trick that spammers like to use is to create one page and then post it in numerous locations, both on one domain and on others. The idea is that if there are enough copies of the page, it will get seen by more people. But search engines don't like duplicate content as it's a waste of space on their servers and does not provide good information to their customers. If a search engine suspects your site is spamming them with multiple copies, your site could be banned.
Don't use robots.txt to ban large portions of your site (AVOID)
In general, using a robots.txt file to keep certain areas of your site off-limits to spiders can be a good idea. But if you ban significant portions of your site (more than half), search engine spiders may mark your site as "forbidden" in general and simply stop spidering your site as often. And if your site is spidered less often, fewer pages will be added to the directory and updated in rank.
Don't write bad or incorrect HTML (AVOID)
Most search engines don't deliberately discriminate against badly coded pages, but if the spider can't read the page because the HTML is bad, then it won't get indexed. Make sure that you validate your HTML regularly and that any issues there are don't affect the page being viewed by a simple user-agent or screen reader.
Don't use frames (AVOID)
Frames and search engines are not a good combination. While search engines are getting much better at reading framed websites, they still don't tend to rank as well as non-framed sites. And even if you get decent ranking, you might not get the clicks because the search engine doesn't know what to display as a title or description of your page.
Don't create Flash splash pages (AVOID)
Search engines can't read images, and they see Flash as a giant image. Flash and search engines don't mix well. If you don't have extensive alternative HTML that displays when Flash is not enabled, then your site won't rank well in search engines. Be sure to test your site with a browser with Flash disabled to find out what the search engine sees. You might be unpleasantly surprised.
Don't write Flash-only sites (AVOID)
If you're going to use Flash on your site, you must have an HTML version that displays when Flash is enabled. It can be tempting to put in just a single line or two of HTML as your non-Flash alternative, after all, you've done so much work on the Flash site. But since the search engines only see the HTML, that's what they'll rank, and you won't rank high with just a tiny version of your site in HTML.
If you must use Flash as your site, and you want to rank well in search engines, you need to be prepared to write your site twice - once in Flash and once in HTML.