Hiya Daniel,
Sounds like you're talking about just adding new buttons or selections to your menu? If that's the case you do exactly what you're already doing for changing your menu, just copy and paste one of the <li> </li> lines and just change the stuff in between to the new selection or if you just want to change what the selection is just change it. CSS menus are pretty easy to manage, and your menu is pretty much already CSS so you're all set to go for that. Styling will be done in the css file for it such as colors, font styles, sizes etc. and the only thing you need to do on your pages is change the text that you want to be on the menu itself as well as the link.
For example a selection of your menu is like this part representing your Products:
<li class="categories_link"><a href="#">Products</a>
<ul id="group_list">
<li><a href="#">Christy Whitman</a></li>
<li><a href="#">Quantum Mind</a></li>
<li><a href="#">Think Right Now!</a></li>
<li><a href="#">Self Improvement Books</a></li>
<li><a href="#">Theta</a></li>
<li><a href="#">EFT</a></li>
<li><a href="#">LifePath </a></li>
<li><a href="#">Abraham</a></li>
<li><a href="#">Quantum Techniques</a></li>
</ul>
Let's say you decide to no longer use the Quantum Mind section, you would just delete the line below completely:
<li><a href="#">Quantum Mind</a></li>
If you want to add a new section then just copy one of the lines and paste it in the location you want it in and then change the href location (where the #'s are at the moment that need links in them yet) and change the actual name of the selection and you're set to go. Very simple to keep it going.
Other than that, if you're wanting to do the menu in one page that updates all the site pages then you'd need to use includes.
If this is still not what you're after let us know