I stumbled across a link to this article on slipstick earlier today while researching some VBA things, and it's so simple I wanted to share it:
How to assign a macro to a toolbar button in Outlook1. View | Toolbars | Customize2. On the Commands tab, click Macros on the left-hand side3. Drag the appropriate macro to the standard toolbar (say, next to the “Send/Receive“ button)4. Right click on the button and change the Name field if you want to shorten the text or assign a different accelerator key5. Click Close
Up until now, when I wanted to assign a toolbar button to a macro, I've been writing code to add the button programmatically, ensure that it doesn't already exist, etc... this makes it a heckuva lot easier.