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

2. Variante:

RewriteEngine On
RewriteCond %{HTTP_HOST} ^www\. [NC]
RewriteRule ^(.*)$ http://domainname.de/$1 [L,R=301]

noch ne andere Variante:

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