you can use include files:
<!--#include file="common_stuff.html"-->
add this line where-ever you want the common stuff to appear, and store the common stuff in the common_stuff.html file, or whatever you want to call it.
in the case of script functions (javascript, vbscript) that are frequently used, they can be stored in an include file, then included at the top of your page and the functions/subroutines can then be called throughout the page.