For several months, I’ve been doing Jim Wendler’s 5/3/1 program. In it, Jim talks about doing exercises for assistance that build muscle mass. He advocates doing several reps of dips and pull-ups every week. At one point, I believe he states we should do 50 dips a week weighted or 100 dips a week with just body weight. If memory serves, he advocates doing something similar with chin-ups or pull-ups.