We found this to be the best way to alter URL for a WordPress URL.

http://codex.wordpress.org/Changing_The_Site_URL

Here you can see we set the DNS at our registrar to our host.

Here are our subdirectories at our host.

Finally, we finish off by creating forwards, that take the main domain and send it to our subdirectories. This allows us to have multiple WordPress blogs all hosted at the same account.