Changing theme for a blog is really tiresome work. If your blog is new you can easily change the theme, where as if you want to change your theme after starting your blog journey then the new theme has to be customized with all the settings. It seems annoying for the visitors and even it may disturb the traffic also. Here comes the use of this important wordpress plugin Theme Test Drive
It allows you to test your new wordpress theme without disturbing the already existing theme. Simply you can test and configure your wordpress theme while your visitors use the default one. Once the complete customization is done, you can apply your new theme to your blog.
Link to download this plugin Theme Test Drive wordpress plugin