create a div around the menu that is set to the exact width of the menu and give the navigation div auto margin on the left and the right.
<div class="navDiv" style="margin: 0 auto 0 auto;">
<div class="menuDiv" style="width:550px">
<Menu Here>
</div>
</div>
this assumes the menu is 550px wide. Remember that total width includes padding.
So if you have 5 menu items with 5px padding on the left and right, and each menu item is set to be 100px wide, your actual menu width is:
(100 + 5 + 5) * 5
or
110 * 5 = 550px