My clients are always working on perfecting the exercises in my program, and the Pull Up is one that is a challenge for both the men and women. Here are some suggestions on how to build up to a perfect, "cheat" free pull up.
The primary muscles used in performing a pull up are, as shown below, the latissimus dorsi, and trapezius muscles.
To perform a pull up you will want to build up strength in these muscles. Along with performing modified versions of a pull up, you will want to do some exercises that will help to strengthen theses muscles.
Here are just a few suggestions for you.
Three Point Row
Body Weight Row
Try to use as heave weight as possible, to build up your upper body strength.
The best way to build up a specific skill to practice that skill, here are some modified versions of the Pull Up that will help you to build up to a cheat free pull up. In doing all of these exercises be sure to lower your body as slowly as possible to get the "negative" chin up or pull up in as well. The eccentric flexion of the muscles will help you to build up your strength much quicker!
Cheater (Jumping) Chin Up
Cheater Pull Up
You can also use an elastic or pull up machine at your gym, or a chair at home. The key is to be sure that you are "cheating" as little as possible, and be weening yourself from the helpers.
You should practice your skill at least once a week, and be sure to work in your other lat and trap strengthening exercises through out the week as well.