16-18ounce gloves and hand wraps every time you hit the heavy bag.
This. (a rare agreement sir!
Honestly. Hitting the bag will always hurt. Even with 16ozers and wraps. As you develop more and more power, your bones won't be able to keep up with your muscles. Eventually the nerves in the skin over your knuckles will deaden and any pain will be minimal. However there will always be some form of discomfort or pain in the knuckles themselves after years, and even months of hard training (this comes from the deterioration of the cartilage from accumulated trauma).
To this day I cannot crack my first knuckle, and if I ever try, pain will shoot down my finger between the first and second knuckle on my right index finger.
Pain, discomfort, and the fear of crippling arthritis is just something someone who hits a bag for years must endure. Its unavoidable as more and more trauma is endured by the knuckles. But smart training, i.e. 16ozers, hand wraps, proper punching, can minimize this damage.