Update: before purchasing additional URLs and testing them, check out the URL rules that Google is currently enforcing (different that when AdWords For Dummies was written):

http://askhowie.com/readers/part-ii-updates/new-rules-about-displaydestination-urls/

The video that was here is misleading, because the new Google rules prohibit the use of display URLs different from the actual landing page domain name. You can still use zoneedit to test subdirectories for the same domain name.

For example:

natural.soapforyourfamily.com

organic.soapforyourfamily.com

healthy.soapforyourfamily.com

Use ZoneEdit for Free – Really!