I'm doing a lot of chin-ups to get that nice v-shape. It's real heavy doing chin-ups but I can do 4 of them now :) I train mostly with a red rubber band that takes of some of the weight so I can do more reps. It's really important to warm up the shoulders before doing chin-ups to prevent injury.
Potty Training Tips do some research (check Pull-Ups Big Kid Academy). Tailor training for your kid (visual/verbal/treats,etc). Expect set-backs. Reward with praise and incentives. Don't force it, wait for signs they are ready.