How to define XAML shapes in a ResourceDictionary and use them

Question

What is the general approach for creating reusable XAML shapes in a ResourceDictionary? If I define a Path or other shape in the ResourceDictionary, how do I go about using it?

I would like to have a clickable diagram in my application, made up of several shapes. I imagine that I would use Buttons in the diagram, but I would like them to look like the shapes I defined in the ResourceDictionary. Sometimes these are non-rectangular.