I just did this myself last week:
1) Set up the domain (e.g. yoursite.com) in the control panel.
2) Create the mirror subdomain (e.g. yoursite.dreamhost.com) and have it point to the domain you set up in Step 1
3) The mirror will point to the directory for the domain you set up in Step 1, so you can ftp/http etc. to yoursite.dreamhost.com and it will take you to your dreamhost account. (And of course when you make the DNS switch later, yoursite.com will go there too.)
That's it! As someone mentioned above, relative links make it a lot nicer for checking out your site before it's public.