Force the www.
RewriteEngine On
RewriteCond %{HTTP_HOST} ^your-site.com [NC]
RewriteRule ^(.*)$ http://www.your-site.com/$1 [L,R=301]
Remove the www.
RewriteEngine On
RewriteCond %{HTTP_HOST} !^your-site.com$ [NC]
RewriteRule ^(.*)$ http://your-site.com/$1 [L,R=301]