One of the wonderful features of ASP.NET is a little nugget called App_Offline.htm. With this one amazing little file, you can take a site down for maintenance for those times when upgrading a site just can't be done with the engines still running.
The way it works is simple: create an HTML page, name it App_Offline.htm, place it in the root of the website, and any request to the site will be redirected to this page. When you're ready to bring the site back, simply remove the file, and presto! The site is back.
The trouble is, this page returns a 404 (file not found) response code in the header, which could lead search engines to drop the site from its index. The page should return a 503 (service unavailable) response code.
Thankfully, someone has already submitted a bug report to Microsoft.