Daniel McPherson who originally brought us URLs Matter has taken it another step and provided great directions for how to reserve those precious top-level URLs when you create a portal. Thanks Daniel!
To add my two cents, after reading URLs Matter some time ago I went and reserved all my top levels that weren’t being used and placed the areas under an Admin area. I then used the Area Contents web part on the admin area page to show a list of the reserved URLs. I additionally added a link list with all of the active top-level URLs.
This little bit of setup can really help you down the road when you are trying to remember what pages in your huge portal have a top-level URL spot and what the URLs are for the areas you reserved for future use.