Quick Way to Take Your WordPress Site Down For Maintenance Without a Plugin
WordPress 3.0 is coming any minute now, which means many of you will be performing some major upgrades in the near future.
Here’s a quick way to take your WordPress site down for maintenance while redirecting to your maintenance page. Add this to your .htaccess page:
RewriteEngine on RewriteCond %{REQUEST_URI} !/maintenance.html$ RewriteCond %{REMOTE_ADDR} !^123\.123\.123\.123 RewriteRule $ /maintenance.html [R=302,L]
Change maintenance.html to wherever you want to redirect your users. Also, update line 2 with your own IP address. This 302 redirect will ensure that search engines do not index your maintenance page as your new homepage.
Source: CatsWhoCode