htaccess redirect all pages to single page

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/

Are you trying to get visitors to old.com/about.htm to go to new.com/about.htm? If so, you can do this with a mod_rewrite rule in .htaccess:

RewriteEngine on

RewriteRule ^(.*)$ http://www.thenewdomain.com/$1 [R=permanent,L]