The only thing that is going to get your pull-up numbers up is DOING them. I used to do pull-ups twice a week (at least 1-2 days between doing them), and always as the first exercise of that session. I also had longer recovery periods between sets of pull-ups than I did with anything else (3 minutes, as opposed to 30-60 seconds), which helped a lot - shorter recovery periods affect reps in each consecutive set. I pretty much went to failure on every set I did, no cheating, elbows fully extended at the bottom of the eccentric portion of the exercise. I started out doing 6 sets, then 8 sets; and then moved to picking a number of reps to get completed (usually 100-120 reps) and just doing as many sets as it took to get those reps in. You're always going to start out being able to do a lot of reps the first few sets, and then your numbers will decrease after that (i.e. I usually manage 15-25 reps the first few sets, and then they decrease down to like 15, 12, 10. 8, etc.).
A note that improving your body composition should also help in increasing your pull-up numbers - I always notice that the leaner and lighter I get, the easier they are to do, and the more reps I can complete per set.