That is a very good question. Search Engine Optimization (SEO) writing is really what anyone who has been writing for the web for a while does; this is just the industry term for it. What exactly goes into proper SEO writing?

Keyword Analysis
There are several tools you can use to do keyword analysis; the only one I use is the Google Keyword Tool. Before you start writing your page or blog posting, you use the tool to develop a short list of short tail and long tail keywords. I usually use two short tail phrases and about three long tail phrases. Short tail phrases are common searches, such as “green living”.

Long Tail Defined
Long tail phrases are more narrowly defined, and would be something like “organic web content writer”. You then use these phrases in your body copy, with long tail phrases at the top and bottom, without making them sound awkward. The keywords shape your copy, you don’t insert them into your copy. You can, but it ends up being more awkward if you do.

Writing in Digestible Chunks
Humans and search engines both like copy that is structured like this article; a headline followed by a paragraph. Don’t make your paragraphs too long either. This also allows readers to skip over areas which they feel they don’t need

Write Decent Headlines
This is especially important for blog posts; your readers are likely using an RSS feed to pick out the posts that they want to read, so make it engaging and don’t make it too short.

There is a lot more to SEO writing, and a lot more to SEO in general. One thing clients have to understand is that SEO does not stop at writing. Decent content is only part of the package; you also need clean code and a link building program. It is also the responsibility of people who do web copywriting for a living to forge relationships with web designers and SEO specialists who can further help clients with these areas of their website presence. It takes a village to make a good website and market it properly.