shutting down a web app for maintenance

Hello All, I'm not sure this is strictly a PHP related question or perhaps a server admin question as well. What do you do when you are trying to shutdown a web application for maintenance (like a membership or registration-required system)? I understand that you can temporarily upload or activate a holding page that prevents users from continuing to login/use the system, but how can you insure that there are no ongoing sessions or users still in the process of doing something? What's the best method for handling this, especially if you don't have full control of the server hosting the web app? Thanks for any advice, - Ben