Force www:
RewriteEngine on
RewriteCond %{HTTP_HOST} ^yourwebsite.com [NC]
RewriteRule ^(.*)$ http://www.yourwebsite.com/$1 [L,R=301,NC]

Force non-www:
RewriteEngine on
RewriteCond %{HTTP_HOST} ^www.yourwebsite.com [NC]
RewriteRule ^(.*)$ http://yourwebsite.com/$1 [L,R=301]