http://httpd.apache.org/docs/2.0/misc/rewriteguide.html
Canonical Hostnames
Description:
The goal of this rule is to force the use of a particular hostname, in preference to other hostnames which may be used to reach the same site. For example, if you wish to force the use of www.example.com instead of example.com, you might use a variant of the following recipe.
Solution:
# For sites running on a port other than 80
RewriteCond %{HTTP_HOST} !^www\.example\.com [NC]
RewriteCond %{HTTP_HOST} !^$
RewriteCond %{SERVER_PORT} !^80$
RewriteRule ^/(.*) http://www.example.com:%{SERVER_PORT}/$1 [L,R]
# And for a site running on port 80
RewriteCond %{HTTP_HOST} !^www\.example\.com [NC]
RewriteCond %{HTTP_HOST} !^$
RewriteRule ^/(.*) http://www.example.com/$1 [L,R]