Oki.. heres the secret for training your pull ups.
Maintain a strict routine everyday to do 20 pumpings at least everyday. Or at least 100 pumpings if possible. I have a frd who maintains this.. boy, he was so good with the pull up.
The best is also to train and practise. Keep on pulling and someday you will break that zero.

I progressed from what they call a 'zero fighter' (those who does zero chin up) to 10. hehehee..
The trick is to keep on pulling.. and be careful, never jerk or kick your leg.. cos it wount be counted. You rather do 5 lesser chin ups then to do 10 with kicks and not counted.
Steady steady man man lai!!