www to no-www, and vice-versa

To redirect requests for www.example.com to example.com (without the www) put this in your .htaccess:

RewriteCond %{HTTP_HOST} ^www\.example\.com$ [NC]
RewriteRule ^(.*)$ http://example.com/$1 [R=301,L]

And to do the reverse (redirect non-www to www), try this:

RewriteCond %{HTTP_HOST} ^example\.com$ [NC]
RewriteRule ^(.*)$ http://www.example.com/$1 [R=301,L]