A talk show is a program for the listener not the callers. If there is not a clear role for callers that advances or deepens the topic, don't use them.
Frame your invitation to callers in a way that will allow you to take the program where you want it to go.
Frame your invitation to callers in a way that will tap expertise and direct experience rather than just opinions. Avoid at all costs asking “What do you think?”
Seek a diversity of caller gender, ethnicity, age and location.
Air more first-time callers. New voices bring new characters and new perspectives that keep your show fresh and interesting