In the latest Webmaster Help video from Google, Matt Cutts discusses the biggest mistakes that a lot of webmasters are making when it comes to SEO.
The top mistake, he says, is not making your site crawlable, which includes not having a domain.
After that, it's not including the right words on the page. Think about what the user is going to type, and include those words, he says. Going further, also have the things that people would be likely to look for on the page. For example, include your business hours. If you're a restaurant, inlclude a menu in plain text - not just a PDF.
The third thing is that people are thinking too much about "link building" when they should really just be thinking about creating compelling content and maketing (including things like talking to newspaper reporters).
The fourth thing is the lack of good titles and descriptions. Make sure you include something that makes people actually want to click on the search result, and something that will be helpful, should users bookmark the site. "Something that lets them know you're going to have the answer they're looking for - something that makes them understand this is a good resource," says Cutts.
Finally, the other big mistake is not taking advantage of webmaster resources, including (but not limited to) Google's own Webmaster Tools. Cutts also suggests following Google's blogs and webmaster videos, attending search conferences, talking to people online (including in Google's forums), and even mentions Blekko's link tool. Basically, just follow the SEO industry.