It should be noted that there is more than one way to test your site before a DNS change. Of the 3 most common methods I think the "mirror" method is the most confusing and requires the most work as the actual DNS change occurs, as noted here.
Calling attention to 2 other methods that I like much better, and why.
Refer to sections 6 and 9 of http://wiki.dreamhost.com/DNS_-_Viewing_site_before_DNS_change
Both of these methods involve telling your local computer to use differnt DNS.
Method 6 is probably the quickest "on the fly method" to use, and in most cases (99%) the one single entry suggested in the example is all you will need for testing your site. In the rare case that you have something more complicated, you will either need to figure out what additional entries you need or use the next method.
Method 9 overrides your normal DNS and uses Dreamhost as the "authority". Normally when you boot up and log into your local computer, the addresses of the "Nameservers" are obtained from you Internet Service Provider that is supplying your local internet service. But if we override this and tell our local computer that we want to use dreamhost's nameservers then we have every entry available to us that dreamhost has generated for the domain.
If you have multiple computers and use one as a development machine, set that one up to use Dreamhost DNS all the time, and leave your other computers DNS set to 'Automatically obtain DNS servers'.
With either 6 or 9 there is no need for "mirrors".