Drop your b-fat percentage is a good way.
I read an article once upon a time in a lifting mag that discussed chins. The record holder for the U.S. Marines built his chinning ability through a rest-pause method. What I took away from the article was that he would hang there, do a rep or two, hang there some more, do a rep or two...etc. I find it sort of unusual since the point of rest-pause is to get around fatigue and doing as many chins as you can is an act of strength-endurance.
If I were doing this I would set up a few metrics. The obviously important one is how many chins you can do repetitively. I'd test that every 1 - 2 weeks to see if I'm getting there. In the workout I would count how many reps I did and what the time between reps was before I reached failure. The objective would be to increase the number of reps to a reasonable number (like 5 to start with). Once I am doing 5 chins with, say 5 second rests between reps I would try to decrease the resting time until I could do 5 continuously.
Colby2152's method is also valid. Do supinated grip pulldowns with whatever weight you can handle. I would do 8 - 10 reps with a 2 second positive and 4 second negative to failure (ala HIT style) because I am familiar with it and I know I can up the weight each session for a few months.
Or just get a well-reputed book on strength-endurance training and take it from there.
-QP
