Test a theme on your blog without showing your visitors
Copy into your plugins folder and activate. You must enable testing on the Theme Tester admin page (Design -> Theme Tester) after which any theme changes by you will not be visible to regular visitors to your blog.
To make any theme changes visible to your visitors you must disable testing and choose your new theme.
Warning! If the theme you are testing writes to blog options that your current theme uses (such as header image or colours), your blog's visitors will see those changes. I don't expect this to happen too often except if you are testing several themes based on the same theme engine such as K2.