Here's a link to a tip I posted on Eng-Tips for adding button separators to toolbars and flyouts that normally don't allow it.  This method will hopefully become obsolete in the next version.  As Anna noted in that thread, the Command Manager already has the ability to "Begin a Group" feature.  So this is only helpful if you're like me and don't use the Command Manager, but want to customize flyouts by changing standalone toolbars.