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.