using mod rewrite so all links contain the www version this code is placed in your htaccess file Not all servers/hosts allow you to use mod rewrite:
RewriteEngine on
RewriteCond %{HTTP_HOST} !^www.yoursite.com
RewriteRule (.*) http://www.yoursite.com/$1 [R=301,L]
OR simply add this line which tells anyone grabbing the other page it has moved permanently to the http://www.yoursite.com
redirect 301 yoursite.com http://www.yoursite.com