I recommend doing two things: pole or rope climbs and secondly doing one-arm assisted chins with a counter weight. First the pole/rope climb. Here I prefer a horizontal pole, a chin up bar is fine too, you want to train going hand over hand across the pole, fast at first and slower and quieter the better you get at it. Next, begin to take longer and longer to re-grab the bar with the other hand, maybe even do a hip slap before grabbing the bar, then slow this down, too. When you can do slow hip slaps your doing pretty darn well and should be getting close to a one-arm chin up. At our Monkey Bar Gym, this is the very sequence I advised one of our trainers, Hadley, to follow. Each time I saw him doing it, he had mastered that level, so I kept making it harder for him. Then a few weeks later, he had mastered that level. This progressed until he could do quite a large number of very slow hip slaps while working his way across our hanging poles. The next week he tried and got his first one-arm chin up!
The second version using a counter balance weight, which is a favorite of another one of our MBG trainers, Tim. Tim is nationally recognized as one of the strongest in the country at chin ups and doing a one-arm chin up at 200 lbs, which he has done, proves just that! How Tim does it is to strap a Jungle Gym through a light kettlebell or power ball and throw the other end over a chin up bar, placing the handle at about shoulder height. Then he grabs the chin up bar with one arm and the one Jungle Gym handle with the other hand. Then he does assisted one-arm chin and pull ups. As the weeks go by you lower the amount of weight you are assisted until you have none, then it’s time to get your one-arm chin up!