All of the above are great tips when it comes to pull ups. I'm a huge advocate for them and see them as a true test of strength, so I try to add them into everything. When I was going through my MOS schools I was terrible at them, and took in all sorts of knowledge from the guys banging out 20-25 at a time, and kept at it until I finally saw the big picture.
Like previously mentioned it comes down to "To get better at pull-ups, you MUST do pull-ups."
Try doing different workouts everyday, or every other day,
for instance try pyramids one day
Max set, 1,3,5,7,9,7,5,3,1, and attempt a pitiful max set following it. Every rep helps, but don't rest for too long.
Following day try 5 pull ups, 10 push ups for as many sets as you can.
Something I found to help me, I don't know how to explain exactly how it helped but put your thumb on top of the bar with the rest of your fingers, I have never gone back to any other grip.
Everytime you enter your room, do a set number, or a max set. It takes time, but I guarantee if you stick to it, you will see the numbers rise over a month span.
Goodluck!