I would definately do assisted chins.
Sometimes after I do 2x10 or 5x5 chins with weight I'll go to the assisted chins machine, and do 50 assisted reps, starting out light and as I fail adding more weight to help me.
That is one of the only vertical movements for my back that gets me good.
And, as I have read in other posts, the only way to get better at something (sprints, certain exercise, etc) is to train that exercise.