There’s two ways, that I know of, to enable Maintenance Mode in Magento.
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):
3. Replace it with this one:
4. Paste the following code above the line you just changed:
$allowed = array('0.0.0.0','126.96.36.199');
5. Change the IP with your own in the $allowed array you just pasted.
You can edit the 503 page by just going to: /errors/default/503.php
I wrote a post Magento Offline Maintenance Extension on 2010.
Check out the Maintenance Page – ArtsOn.IT extension.