CGC Maintenance Mode Pro is a plugin for WordPress that is designed to make it extremely easy to safely perform upgrades and maintenance on your site.
While in maintenance mode, unauthorized users are sent to either a WordPress page, external URL , or custom template. Authorized users, specified by IP, are permitted to see the site as normal.
Maintenance Mode can be turned on or off at any time, and can even be set to turn itself on and off at a specified date and time.
Rest easy next time you need to perform upgrades by using this plugin to send your unauthorized users to a maintenance page, letting them know what is going on.