Here is an article that might help you out.
If you don't want to follow the program...
It still attacks common problems for why you can't advance and how you can start.
If you do want to follow the program...
If you find it to challenging or need to build up to the intense style of the program just take out day 6 and make day 6-7 complete rest days. Otherwise it is solid program to help you increase pull up ability.
(Site seems to be a little unstable right now but give it a few mins and it should pop back up)
MUSCLE WITH ATTITUDE - Ladies, You Can Conquer the Pull-Up