Following on from my recommendation of using the Theme Tester plugin when working on live sites, this next plugin will essentially help you to replace the standard login form provided by WordPress.
Once Login Logo by Mark Jaquith is activated all you’ll need to do is place a file named login-logo.png within the wp-content folder of your WordPress install, what makes this plugin even more useful is that a recent update meant that this plugin is now compatible with WordPress multisites meaning each site can display a custom logo on the login screen.
Below shows the results we achieved when installing the plugin on our site.