apache httpd-vhosts.conf

<VirtualHost *:80>
    ServerAdmin [email protected]
    ServerName localhost
    ServerAlias *.localhost
    VirtualDocumentRoot /Users/mikekilmer/Sites/%0
    php_admin_value auto_prepend_file /Applications/MAMP/conf/apache/extra/setdocroot.php
# content of above file 
# $_SERVER['DOCUMENT_ROOT'] = str_replace($_SERVER['SCRIPT_NAME'], '', $_SERVER['SCRIPT_FILENAME']);
    RewriteLogLevel 3
    RewriteLog "/Applications/MAMP/logs/rewrite.log"

    <Directory /Users/mikekilmer/Sites>
       Options All
       AllowOverride All
       Order allow,deny
       Allow from all
    </Directory>
</VirtualHost>

In the http.conf file one directory above this, uncomment the line:

# Virtual hosts
Include /Applications/MAMP/conf/apache/extra/httpd-vhosts.conf

around line 571.

See http://stackoverflow.com/a/30176719/2223106