First post so please be gentle :)
When creating user controls in ASP.NET MVC, what is the best way to structure the code so that the controllers that invoke views that u
Have a look at RenderAction and RenderPartial. Those are the canonical ways to arbitrarily inject a common control into a view.
Use RenderPartial when you want to include the data as part of your ViewData infrastructure.
Use RenderAction when you want the data to be separate from the ViewData infrastructure. The data will come from the controller method you specify in RenderAction.
Check out the NerdDinner tutorials, if you haven't done so already.