If you have a website you are going to have to get good at search engine optimization if you want people to find it. If your site doesn't rank on the first page of the search engines you can be sure that you won't get many visitors. SEO isn't really all that complicated but a lot of people still get it wrong. The biggest SEO mistakes are the ones that people seem to make over and over again.
Given that the most important factor in SEO is building links to your site it should come as no surprise that the biggest mistake that people make involves links. Probably the biggest mistake is that they don't use the proper anchor text in the links that are pointing at their site. In most cases the keywords that your site will rank for are the ones that you are using as the anchor text in the links. So many people build links with things like "click here" or "visit my site" as the anchor text. Unless you those are the keywords you are trying to rank for you are making a mistake when it comes to SEO.
A related mistake that people make is when they name their site. A lot of people name their site after themselves or they try to come up with some clever title for their site. This is a mistake; you need to use the keywords that you want to rank for in the title of your site. There are a few reasons for this. The biggest is that when people are linking to your site they will usually use the title of your site as the anchor text. As mentioned above it is important that the anchor text include your keywords. The other reason that you want to have your keywords in the title is that it makes it easy for the search engines to know what your site is about. The title is still the biggest factor that the search engines use in determining what your site is about so make sure that the title includes your keywords.
Another major SEO mistake that people make is not having good internal linking structure. A lot of people get so focused on finding links from other sites to their site that they forget that a link from one page on your site to another still counts as a link. Admittedly they aren't as valuable as an external link but they are also a lot easier to get. Equally important a good internal linking structure will make it easier for the search engine spiders to crawl your site and make sure that all of your content gets indexed. Take a good look at your site and make sure that any pages that are related are linked together; you will be surprised at how much this will help your ranking.