Here are 5 ways to use includes to create common content and have that content automatically appear on the website.

• Server Side Includes (ssi)
• PHP
• JavaScript
• iFrame
• XML + XLST

Note 1: to use SSI, your web server needs to have server side includes enabled.

If it isn't already, add these lines in the .htaccess file:

AddType text/html .shtml  
AddHandler server-parsed .shtml .html .htm .shtm 
Options Indexes FollowSymLinks Includes

Note 2: to use PHP, your web server needs to have PHP enabled.

And here is the Including Files document with the code for the index pages:
IncludingFiles.pdf