What does Personal Interaction have to do with effective software development? Lots! As Jerry Weinberg says, "It's always a people problem." As the Agile Manifesto states, "Individuals and interactions over processes and tools."
Effective teamwork starts with effective interaction between individuals.
Techniques to improve interaction
For further learning
AYE Conference -- Amplifying Your Effectiveness
Secrets of Agile Teamwork by Esther Derby and Diana Larsen.