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.