Create a decent “sorry this site is down for maintenace” page with a “have a Nice day” message then when you do need to upgrade things use the following htaccess rewrite rule to forward all traffic to this page.

Adapted from Marco’s Webdev Notepad

RewriteEngine on
RewriteCond %{REMOTE_ADDR} !^83.57.178.37$
RewriteRule !maintenance/index.html /maintenance/index.html [L,NC,R=302]

From Ask Apache (I like)

ErrorDocument 403 http://www.sub.domain.com/
Order deny,allow
Deny from all
Allow from 83.57.178.37