Here's how I train to get 'harder' punches.
Punch a wall-bag over and over again. Apply dit ja jow. Repeat.
At my school we have two wall-bags, one filled with rice, one with rocks. I started on the one with rice, then worked my way up to the rock-filled bag after about 8 or 10 months.
Here's a drawing of a wall-bag, or 'sao bao' that I found. http://www.kungfu-videos.com/sao_bao_sketch.html
The point of this kind of training is to get your hands accustomed to punching hard objects, also to strengthen your wrists and punches in general.
So, do other schools have this kind of training? What different methods are there for getting similar results? I've heard of karate-style makiwara training, but don't know any details about it.