This a very common query which is brought up a great deal of the time.

How do I update one file to change my WHOLE website?
or
Whats the easy way to update all my pages with one change?

For example,
You have 50+ pages in your website, and want to add another button to your navigation menu. Instead of going through all 50+ pages and changing your navigation, how would you update one file to update the rest?

Well, it's a simple answer actually.
Server Side Includes (SSI).

Simply creating one file containing your HTML code for the navigation menu, and adding one line to your page, will result in an easy way to update all your files.

There are many ways, such as PHP or ASP.

PHP

for any page you want it to appear

PHP Code:

Your HTML code here

<?php include('navigation.php'); ?>

More HTML here

and inside navigation.php (same directory for this example)

PHP Code:

<div>
Whatever
</div> 


ASP

ASP has the same concept, but just different coding to do say

for any page you want it to appear

HTML code here

<!--#include file="navigation.inc"-->

More HTML code here

and navigation.inc would typically contain

<div>
Whatever
</div>

Getting it to work
For the above two examples to work, the files would need to be saved in either *.asp format or *.php format, either that or you could add a line in your .htaccess file

http://www.javascriptkit.com/howto/htaccess4.shtml

That is a great tutorial for this.

Sorry, but oversimplified and inaccurate on all but the actual code for the PHP include().

A somewhat more comprehensive walkthrough can be found at
Using includes
with;
SSI
ASP
PHP

</self-promo>

I take the reverse approach.
I use templates so, none of the pages know about the decoration.

Originally I did this by implementing a servlet filter to intercept the request, and apply the filter. I've written (and debugged) this so many times in so many places. Now I use Open Symphony's SiteMesh, which is a pretty good implementation.
It can use browser specific templates, useful for legacy systems and pages for phones. It even supports localized templates, so you can use location specific designs. The example always given is, the color red is associated with happiness in most eastern countries, with danger/alert in western ones. It's easily extended too so, you could have a different template for logged-in people vs guests, etc...

The problem with includes (besides exposing information where it's not needed) is, what if you want to change the include, or add another one? You can still be left editing 100+ pages....
Another cool thing with a template filter, is can works with generated pages. I might have a special case servlet (instead of a JSP) that just outputs HTML. A template can handle it. Includes would require code edits and a recompile.