I’ve mentioned it a few times around the forums - there are a lot of exercises that get carryover from other exercises. Pull-ups, to me, are one of the few exercises that get virtually no carryover from anything else. You need to do tons and tons of them over the course of months and years. There are people who can pull the whole stack on a lat pull down and row tons of weight that can’t do 5 perfect pull-ups.
There are three methods that helped me get great at pull-ups.
Method 1: Pick a number and do that number of pull-ups every day. Let’s say it’s 30, to start. It doesn’t matter if you do six sets of five or fifteen sets of 2, just get them in every day. Increase the number by 10 when it gets to be easy.
Method 2: Do 1 pull-up, rest, do 2 pull-ups, rest, and as far as you can go up. When you hit 10, go back down to 1. 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1 pull-ups = 100 pull-ups. When you can complete this workout, you will be good at pull-ups. I still do this, but I superset Bench or Shoulder presses or pushups with the same rep scheme.
Method 3: Adding weight. This is where things can get risky. There are a lot of different variations that can add difficulty to the BW pull-up, but the fact is that if you can slowly add weight to your body, then once can do a few pull-ups with 2 plates hanging from you (I think 4 is my max with +90), you will be able to bust out lots of bodyweight reps. The only problem is that the shoulder is already in a compromising position, and adding weight can be risky for certain people.
A few extra pointers:
Do your pull-ups wider than shoulder grip and with an overhand or neutral grip most of the time. Underhand is fine, but lots of people end up doing arm-ups and get elbow problems.
Do your pull-ups dead-hang. Get full scapular protraction at the bottom and initiate with scapular retraction. Get your chin just to the bar, and don’t worry about getting too high up.
Finally, I’m not a huge fan of the assisted pull up. When people can’t finish sets, they turn to that machine, but the stability of having your feet on any sort of support completely changes the motion. If you add high volume pull-ups to your workouts and can’t finish the reps, do negatives (jump up and let yourself down slowly).