i went from doing 13-14 pulls up to 26-27 pull-ups largely by doing them with weight.
oh, and do them with palms facing outward. some people find this more difficult at first, but i will almost guarantee that it will pay dividends in the long run. you almost never see a pull up monster who does them with palms facing in. i think i've seen maybe one person.
I train back one day per week, and I always start with pull ups. Sometimes I do a pyramid, sometimes I just do a bunch of sets of 10, and sometimes I do them with added weight (when I was really training to increase my pull ups, I almost always did some with added weight. At one time, I could do 10 with a 45 lb weight chained to my waist. doubt i could do that right now, though.)
Also, do lat pulldowns (concentrate on pulling your ELBOWS down, not your HANDS down. if you concentrate on pulling your elbows down, your hands and the bar will follow, and it will help you target your lats instead of your biceps) and T-bar rows.
Low cable rows, seated rows, bent-over dumbell rows, or any other back exercise is also good. Do just as much for back as you do for chest...I do about 15 sets (3-4 sets of 4 different exercises, or occasionally 5 sets of 3 exercises) for either of those workouts, and I hit them both once per week.
oh, and you need to get down in the 6-10 rep range to build strength when you're doing the weighted pull ups. if you get so strong that you really aren't straining at all to do pull ups, you won't really fatigue until you're almost done (when i do them, it starts to burn around the high-teens, and i don't stop until i hit 20. then i hang there for a few seconds and do some more).
however, you should also do some max sets sometimes to work on muscular endurance (although i've found it to be easier to just get so strong that you aren't really straining than to build up lots of muscular endurance so that you can strain for a lot of reps, if that makes any sense)