Sooner or later you will make a decision that disappoints someone, and email will make it worse. In person is best and a phone call is second. Knowing you have to face the person also makes you think harder about the decision itself. This is the lesson I find hardest in the whole book, and I am still working on it.
This is the short version. The full lesson, with the story behind it, is in the book.
Take The Summer Camp MBA with you, or get the field notes by email while you work through the lessons here.