I just did this on a client site. I set up my rules like this:
RewriteEngine on
RewriteCond %{HTTP_HOST} ^(.*)fsdegrees.com [NC]
RewriteRule ^(.*)$ http://www.56degrees.co.uk/$1 [R=301,L]
The first rule checks the incoming domain and verifies that it is the old domain. The second sends a 301 redirect and visitors are sent to the corresponding page on the new domain, assuming that the URL structure has remained the same. You do not want to just serve up a 404 page as you will lose all of the indexed links in search engines. 301 redirects will tell search engines where the new replacement pages are. (You’ll get 404 ONLY if the new pages cannot be found.)
And these rules will work whether both domains point to the same folder or if they point to different folders. In the case that they point to different folders, you will have to add these rules to an .htaccess file in the old domain folder.