How To Do Variable Variables

I have a project which needs some pulldown lists that the build submitter can choose from. The first list is the product. The second list is a set of customers. The customer list is dependent on which product is selected from the first list. If these two lists are independent, then I'll need to write some code that makes sure the build submitter has made some valid choices. I'd rather not do that by restricting what choices can be made in the first place.

How do I make one pulldown variable list dependent upon the choice made in another?

Thanks,
John Bobinyec