Decide what the purpose of your product or campaign is first, then design it in such a way that it facilitates the purpose.
I run into this frequently in web design. When I ask what the page is supposed to accomplish, I usually get a list of items the customer wants on the page. Or I get descriptions of being cool, flashy or professional.
The first challenge is to decide what the primary function of the site is. The form then facilitates and enhances the functionality.
So think function first then determine the best form to get it done.
How can you use the form follows function principle in your business?