Rewrite rule to add missing www
RewriteEngine on
RewriteCond %{SERVER_NAME} !^www\..*$
RewriteRule ^/(.*)$ http://www.%{SERVER_NAME}/$1 [L,QSA,R=302]
From the manual, using %1 as a back reference from the RewriteCond in the RewriteRule should work, but it didn't on my installation. Weird.