An appropriate way to start off SEO posts is a short on on robots.txt. This is the file that web crawlers and bots are supposed to look to for instructions when indexing your site, including an instruction not to index. Apparently there are some naughty bots out there that ignore any type of instructions but the big players like Google attempt to follow the robots.txt directions.

For a quick review, check out http://www.robotstxt.org/ They list examples including this one that specifies directories not to index.

User-agent: * 
Disallow: /cgi-bin/ 
Disallow: /tmp/ 
Disallow: /~joe/