I have a site that I have been updating several times a week for 6 months now but I still do not seem get any traffic from Google (or MSN or Yahoo either).

I have been using keywords in headers and content as naturally as possible, I probably under optimise rather than over do it.

On some occassions I have had the odd visitor from Google but when I check the referrer path my listing is not in Google. It seems to appear there for less than a day andthen completely dissapears never to be seen again.

Is it possible I am still sandboxed?

Nope it will be the aging delay. "bouncing" in and out of datacentres seems to be one characteristic of that.
The "sandbox" is where you are indexed but only rank for very low competition phrases.

the odd visitor you get from google could be the google bot spidering your pages. Start trading links and increasing your content. This should help. If not consult a professional.

Time will improve your ranking, keep updating as you are and like Guerrilla said, "Start trading links and increasing your content." Also, if you do decide to seek a professional (SEO Company). Make sure you get references and search forums before you "buy".

Originally Posted by Guerrilla

Start trading links and increasing your content. This should help.

its a good start to improve visibility of your site..

Submit your site to as many free and search engine friendly directories as you can find.

Also it will help to research as many SEO Forums as you can find. Some of the forums have great sections providing a step by step SEO Guide.

Happy SEOing