How is it that I forget the steps to accomplish such a simple task so frequently? Do I have a poor memory, or is Visual Studio's method for adding separators beween toolbar buttons just plain unintuitive? I blame Visual Studio.
Here are the steps to once and for all prevent me from forgetting
1. Open the Customize dialog box for toolbars.
2. select a button that is already present in the toolbar. The Modify Selection drop-down button should become enabled.
3. Its pop-up menu has a Begin a Group check item at the bottom; use that to insert a separator before the button.