Paolo RedaelliTheme Test Drive WordPress Plugin allows you to safely test drive any theme on your blog as administrator, while visitors still use the default one.
It happens completely transparent and they will not even notice you run a different theme for yourself.
Best part is you can even set the testing theme options (if it has them) in the Admin panel while you are testing the theme.
You can also preview any theme by adding “?theme=xxx” to your blog URL. For example http://www.myblog.com/?theme=Default