RewriteEngine on

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

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