Simple approach to making your website show an offline message while you perform maintenance:

ErrorDocument 503 "Our website is temporarily closed for maintenance."
RewriteEngine On
# TO ALLOW YOURSELF TO VISIT THE SITE, CHANGE 111 222 333 444 TO YOUR IP ADDRESS.
RewriteCond %{REMOTE_ADDR} !^111\.222\.333\.444$
RewriteRule .* - [R=503,L]

Source:
http://25yearsofprogramming.com/blog/20070704.htm