Magento: Take your site offline to customers, but retain access to the backend yourself

Create html file "maintenance.html" in Magento folder.

copy and paste the following lines to the top of your .htaccess file:

Order deny,allow
Deny from all
Allow from 127.0.0.1

Order allow,deny
Allow from all

ErrorDocument 403 /maintenance.html

Change IP 127.0.0.1 to your own.

Further down in the same .htaccess file, change

############################################
## By default allow all access
Order allow,deny
Allow from all

to

############################################
## By default allow all access
#Order allow,deny
#Allow from all

This will redirect everyone except yourself (using your specified IP address) to the maintenance.html file.

Further down in the same .htaccess file, change

############################################
## By default allow all access
Order allow,deny
Allow from all

to

############################################
## By default allow all access
#Order allow,deny
#Allow from all

This will redirect everyone except yourself (using your specified IP address) to the maintenance.html file.
Note: These instructions assume that you have a standard webserver setup. If your webserver serves pages directly to the visitor and doesn’t sit behind another server (eg. Varnish), this could change the IP address a request appears to be coming from and the above code won’t work. It also assumes that your store complies with Magento’s system requirement “Ability to override options in .htaccess files”.

From Magento version 1.4 onwards, you can place your store into maintenance mode by creating an empty file called “maintenance.flag” in your Magento root folder. Visitors to the site will be greeted by a “service temporarily unavailable” notice. Unfortunately, the maintenance flag also makes the site (including the backend) unavailable to you, so we don’t recommend this option.

Bring your Store Back Online
If you took your store offline before installing the extension, bring it back online. To do this, delete the following lines from the top of your .htaccess file (where 127.0.0.1 is your own IP address):

Order deny,allow
Deny from all
Allow from 127.0.0.1

Order allow,deny
Allow from all

ErrorDocument 403 /maintenance.html

Further down in the same .htaccess file, change

############################################
## By default allow all access
#Order allow,deny
#Allow from all

to

############################################
## By default allow all access
Order allow,deny
Allow from all

Your store will now be back online.

http://cdn.fooman.co.nz/media/custom/upload/TheUltimateGuidetoInstallingMagentoExtensions.pdf

or use this solution: Maintenance mode Magento CE