WPCodeBox has a maintenance mode snippet in the snippet repository.

To add this code snippet to your WordPress site, just open the Code Snippet Repository, search for "maintenance", or look under the "maintenance" category. And select the "Enable maintenance mode for non-logged-in users". Once you downloaded the snippet you can just enable it each time you want to put your WordPress site in maintenance mode. The site will be in maintenance mode just for non-admin users.