How to put a site offline in WordPress
Install the Site Offline or Coming Soon plugin found by visiting this link http://wordpress.org/extend/plugins/site-is-offline-plugin/
Use this plugin to enable/disable site offline mode. When Site offline Mode is enabled regular visitors of your site will see a page(you can edit that page by going to settings->Site Offline Mode) and you as a admin can make and test safe changes to your site. Very helpful for those developers who need coming soon pages for there sites.