As you might have deduced from my CV, I have some experience in designing and developing IVR applications.
Basically, they must be attractive to the callee so the usage is convenient and intuitive. Going forward, the callee should get the impression like they are talking with a human being, not an application, which is not that easy 
Recently, I’ve come across an interesting series of posts on The VUI Post weblog suggesting how a dialog should be closed. So far there are 16 pieces of advice, perhaps new ones will appear (will track it).
Here are the links:
If you’re interested in IVR apps, I recommend reading that!
