Add "noindex,nofollow" Meta-Tag to Magento Backend Login.
Allow to double secure the admin login with a HTTP-Auth prompt. Add a IP Whitelist to prevent the HTTP-Auth for sertain IPs.
Create two files in the magento root folder namend "secureadmin_ip.txt" and "secureadmin_auth.txt". Add your allowed ips line seperated into ""secureadmin_ip.txt". Add your users for the HTTP-Auth line seperated into "secureadmin_auth.txt" with the format "name:password" e.g. "admin:admin123".
Add this to your .htaccess
<Files secureadmin_ip.txt>
order allow,deny
deny from all
</Files>
<Files secureadmin_auth.txt>
order allow,deny
deny from all
</Files>