Two quick ways we've done similar in the past, one is to use ordering to define it, so where you put a number in the admin section to determine the order it is shown, use a range to define one menu and another range to define another. So you could start the first menu at 0, the second menu at 100, third at 200 etc. The code is then fairly simple to only pull the ones out where the number is less than 100 etc.
The other way is to make the headings categories, and remove the link from them in the tpl file. You then need to set it to show subcategories all the time, to do that have a read of this:
http://forum.x-cart.com/showthread.php?t=45126 (third option)