$ip = $_SERVER['REMOTE_ADDR'];

$allowed = array('1.1.1.1','2.2.2.2'); // these are the IP's that are allowed to view the site.

if (file_exists($maintenanceFile)) {

if (file_exists($maintenanceFile) && !in_array($ip, $allowed)) {

// account for load balancer that passes client IP
if (isset($_SERVER['HTTP_X_FORWARDED_FOR'])) {
   $ip = $_SERVER['HTTP_X_FORWARDED_FOR'];
}
if(empty($ip)) {
    $ip = $_SERVER['REMOTE_ADDR'];
}

// whitelist your ips
$allowed = array();
$allowed[]='WHITELIST.IP.ADDRESS.#1';
$allowed[]='WHITELIST.IP.ADDRESS.#2';

if (file_exists($maintenanceFile)) {
    if (file_exists($maintenanceFile) && !in_array($ip, $allowed)) {
        include_once dirname(__FILE__) . '/errors/503.php';
        exit;
    }
}

if (file_exists($maintenanceFile)) {
    include_once dirname(__FILE__) . '/errors/503.php';
    exit;
}

if (file_exists($maintenanceFile) && strpos($_SERVER['REQUEST_URI'], '/admin/') === false) {
    include_once dirname(__FILE__) . '/errors/503.php';
    exit;
}

RewriteEngine On
ErrorDocument 503 /errors/503.php
RewriteCond %{REMOTE_ADDR} !^4.3.2.1 [NC] #your IP
RewriteCond %{REMOTE_ADDR} !^4.3.2.2 [NC] #other IP if needed
RewriteCond %{REMOTE_ADDR} !^127.0.0.1 [NC] #localhost maybe needed depending on server setup
RewriteCond %{REQUEST_URI} !^/errors/503.php
RewriteCond %{REQUEST_URI} !^/media/
RewriteCond %{REQUEST_URI} !^/images/
RewriteCond %{REQUEST_URI} !^/css/
RewriteCond %{REQUEST_URI} !^/js/
RewriteCond %{REQUEST_URI} !^/skin/
RewriteCond %{REQUEST_URI} !^/index.php
RewriteCond %{REQUEST_URI} !^/admin #your admin path
RewriteCond %{REQUEST_URI} !^/admin/
RewriteRule ^(.*) http://www.yourwebsite.com/errors/503.php [L,R=503]