If your aim is to redirect all pages to a single maintenance page (as the title could suggest also this), then use:
RewriteEngine on
RewriteCond %{REQUEST_URI} !/maintenance.php$
RewriteCond %{REMOTE_HOST} !^000\.000\.000\.000
RewriteRule $ /maintenance.php [R=302,L]
Where 000 000 000 000 should be replaced by your ip adress.
Source:
http://www.techiecorner.com/97/redirect-to-maintenance-page-during-upgrade-using-htaccess/