Well, I commonly do it by placing the header (up to the point where you declare your content cell) in a header.php or whatever...then you just include it in your page:
PHP Code:
<?php
include('header.php');
?>
You do the same with the footer. Place all the code after your content cell in footer.php or whatever...then use the include only footer.php or whatever you named your file.
When you need to make a change, you do it to the header or footer file..and it affects every page that includes those files.
Of course that method requires PHP, and may not be the best way for you. Just a suggestion. Hope that helps.