Add the below to your .htaccess file in order to have your-domain redirect to www.your-domain. Just make sure where it says “your-domain.com” to fill in your domain on both lines 2 and 3.

RewriteEngine on
RewriteCond %{HTTP_HOST} !^www.you-domain.com$
RewriteRule ^(.*)$ http://www.your-domain.com/$1 [R=301]

Why would you do this? It makes your site nice and clean and loading off www.your-domain.com. In addition this avoids the duplicate content penalty from search engines, because otherwise your site appears to have a site at “www.yoursite.com” and at “yoursite.com”.