I put a 25# weight on a pulley, and slowly increased the number of reps where I grab the weight with one arm, and use the dominant arm to pull the remainder of my body weight. Over time, because you can only pull with the supporting arm a total of 25# or so, you are lifting the majority of your bodyweight with one arm. Increase the number of reps, and eventually you can reduce the amount of weight on the pulley. I used my Xtreme (gymnast) rings as pulleys and hooked them over the perpendicular grips of an Iron-gym setup in the basement. It took me about 4 weeks with this technique, and once I got up to 5 reps with 25# on the pulley (each arm, for 3 or 4 sets), I have now been able to perform a couple one arm pullups, as a result of this pulley progression to help me get there. I will also start negatives and lock-offs as suggested above, as well as decreasing the weight on the pulley to 15# or less, so I can change things up a bit, develop more stabilizing strength during the decent of the OAP, and try to increase my reps. Also during this same few weeks, I've been working on alternating one arm pushups, and that has strengthened my back to stabilize, which I believe has had a synergistic effect on my back for the OAP. I can't ever know for sure, but I thought I'd mention it, just in case this method alone doesn't get you there.