The Call to Discipleship: The Producing Principle

John 15:1-17 is where Jesus speaks of the second "test" of whether or not we are truly His disciples - especially verse 8: "This is to My Father's glory, that you bear much fruit, showing yourselves to be My disciples." Simply put: we are to be producing much fruit for God - fruit that will last, Jesus says in verse 16. What does this mean? How do we become "fruitful?" What is this "fruit" we are to produce? How do we measure up to this "test" of a genuine, authentic disciple?