Quite some time ago now I wrote about the importance of 301 redirects for SEO purposes.
In a minor addition to that post here is a snippet I often use to ensure search engines do not view http://yourdomain.com and http://www.yourdomain.com as duplicate content.
To perform a redirect from domain.com to www.domain.com, insert the following code into your .htaccess file.
# mod_rewrite in use
Options +FollowSymlinks
RewriteEngine On
RewriteCond %{http_host} ^domain.com [NC]
RewriteRule ^(.*)$ http://www.domain.com/$1 [R=301,L]
To perform a redirect from www site to non-www site, use the following code in .htaccess file.
# mod_rewrite in use
Options +FollowSymLinks
RewriteEngine on
RewriteCond %{HTTP_HOST} .
RewriteCond %{HTTP_HOST} !^domain\.com
RewriteRule (.*) http://domain.com/$1 [R=301, L]