SEO (search engine optimisation) simple means optimising on page text for relevant keyword weight, frequency and prominence to the html and other content of a web page/site for targeted natural listings. The idea is to have better SEO than competing websites and thus rank higher and more prominently. SEO provides a cheaper long term solution for increased qualified traffic and generates customer enquiries that ultimately convert to sales.

One of the features of my SEO is to try to identify your uniqueness and use SEO to promote it. In theory SEO is easy and more about a 'common sense' than a science.

There are thousand of SEO companies who are PAYING to appear in the sponsored listings of search engines i.e. Google Adwords. Why anyone would choose to use the services of these charlatans is beyond me. If they cannot SEO their own website, why would you give them your money?

My SEO is totally self taught based on a commonsense perspective, and ANYONE can learn to do it. I am willing to teach you to. I will give you my SEO diy guide, but I will need a quality link from you in return.

Some useful questions to ask an SEO include:

Can you show me examples of your previous SEO work? 
Do you follow the Google Webmaster Guidelines? 
What kind of results do you expect to see, and in what timeframe? 
What's your experience in SEO industry? 
How long have you been in business?

What's an SEO? Does Google recommend working with companies that offer to make my site Google-friendly?
http://www.google.com/support/webmasters/bin/answer.py?answer=35291

I recently stumbled on a great tip for websites which do not have a sitemap. As you probably know it is very important to the SEO to make sure all your pages can be found, and a sitemap is critical for this purpose.
Google prefers a sitemap in xml and here is a website that generates one for you, which you can then download and add to your server.
http://www.xml-sitemaps.com