How to hide Admin Login Page

Basically this is possible in 2 steps:

1. Rename the current folder admin to whatever you want
2. Edit ../admin/config.php and replace all values admin with the previous renamed folder name

Now you can access your backend via the URL http://yourdomain.com/NEWNAME.

But, be careful in future: everything (new releases, modifications) have this original folder name (admin).
Before you want to install any modification, rename first this name to your new otherwise the installation (copying) will fail.