Say you need to move your web servers to new data center, or upgrade some hardware or perform any other task that requires to power off your web servers. Here’s a solution that is recommended by most SEO’s and Google:

Configure the server to return a status of 503 (network unavailable), you can also add the Retry-After response-header field to tell bots when to come back.

If you use PHP scripting language something like this should work:
<?php
header("HTTP/1.1 503 Service Temporarily Unavailable");
header("Status: 503 Service Temporarily Unavailable");
header("Retry-After: 3600");
php?>

Another thing – you shouldn’t use 301 (or any other) redirects. I mean if someone comes to yoursite.com/somepage.html this page should display the 503 not redirect to some yoursite.com/503.html page.

As for content you can show something like this:

<title>503 Service Temporarily Unavailable
<h1>Service Temporarily Unavailable</h1>
<p>Welcome to YourSite.com, we're currently under maintenance and will be offline until SOME DATE & TIME.
Please try again later.</p>

You can also add your logo or any other content you wish to show your visitors while your site is down.