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 AdSense to boost your rankings (AVOID)
AdSense is a way to earn money on your website. But contrary to popular believe, having AdSense ads won't improve your ranking in search engines, even Google. They won't hurt your rankings either. It's perfectly fine to use them, but don't expect them to improve your search rankings.
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 create pages of links (AVOID)
Pages of links are boring both for your customers and for search engines. Most search engines value links that are in context and appear related to the page as a whole. Note, however, that many social networking sites (like Digg and del.icio.us) tend to favor pages that are lists of links, so sometimes it can be advantageous to write them anyway, just don't expect them to rank high in search engines.
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.