There's two ways, that I know of, to enable Maintenance Mode in Magento.
Maintenance.flag
Since the version 1.4 you can activate the Maintenance Mode by just creating a file called maintenance.flag in your root directory.
Downside: it gets activated for everyone, including you with the admin sesion.
Don't panic! There's a workaround, here it is:
1. Open your index.php file in the root folder.
2. Look for this line (should be around the 57th line):
if (file_exists($maintenanceFile)) {Code language: PHP (php)3. Replace it with this one:
if (file_exists($maintenanceFile) && !in_array($ip, $allowed)) { Code language: PHP (php)4. Paste the following code above the line you just changed:
$ip = $_SERVER['REMOTE_ADDR'];
$allowed = array('0.0.0.0','1.1.1.1');Code language: PHP (php)5. Change the IP with your own in the $allowed array you just pasted.
What more?
You can edit the 503 page by just going to: /errors/default/503.php
Extension
I wrote a post Magento Offline Maintenance Extension on 2010.
Check out the Maintenance Page - ArtsOn.IT extension.