Short and sweet.
<IfModule mod_rewrite.c> RewriteEngine On RewriteCond %{HTTP_HOST} !^www\. RewriteRule ^(.*)$ http://www.%{HTTP_HOST}/$1 [R=301,L] </IfModule>
And if you want to remove the “www” portion, just change the condition (RewriteCond) and rule (RewriteRule)…
You may ask “Why should I do this?” and the asnwers are simple. It promotes linking uniformity and ensures you don’t end up being negatively affected by having “duplicate content” across (sub)domains in search engine indexes.