weak when it comes to doing pullups....

[bifteck]

Do it like nate says, that's the only way I increased my own rep amount

 

[Lord_Suston]

I do a high rep set and some lat pulldowns with more weight than my own, plus barbell rows definitely help

 

[OfWolfandMan]

Try wrapping a band(like the westsiders use..)behind your back,and under your arm pits,then onto the pull up bar.You can adjust the resistance,by making the band more or less loose.
Ive also noticed that when I added some weight,I started feeling stronger at pull ups than when I just used bodyweight...It felt better than just pulling my own bodyweight for some reason.I also think that there is a groove that you get in to that makes it easier