this is what I'm using on my site:
DirectoryIndex index.php
<IfModule mod_rewrite.c>
RewriteEngine On
# Rewrite all domains to one domain
RewriteCond %{HTTP_HOST} .
RewriteCond %{HTTP_HOST} !^www\.gute-mathe-fragen\.de [NC]
RewriteRule (.*)
http://www.gute-mathe-fragen.de/$1 [R=301,L]
# Rewrite for neat URLs
RewriteBase /
RewriteCond %{REQUEST_URI} ^(.*)//(.*)$
RewriteRule . %1/%2 [R=301,L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^.*$ index.php?qa-rewrite=$0&%{QUERY_STRING} [L]
</IfModule>
# and it works: try without www:
http://gute-mathe-fragen.de