I found a very cool plugin that allows you to test out and configure themes for your self hosted wordpress blog behind the scenes. Your site visitors will keep seeing your current theme until you are ready unveil the new one. The Theme Test Drive plugin is made by Vladimir Prelovac and is free to use. I love it because of how well it works and it's very easy to use. If you use Wordpress then I highly recommend you to take a look at it and try it out.