Try almost anything:
Starting off with two sets of 5 with 3or5 mins rest in between and work up to 5 sets. Then start again with two sets but with less rest and work back up to 5 sets.
Or start again with 7 pull ups per set.
Or do 2 pullups a minute for 5 minutes and build up untill you can do it for 10 minutes, then start again with 3 pullups per minute and so forth.
I think that it doesn't really matter where you start. If you plan how you intend to improve then it is relativly easy to see if you are finding it too hard or too easy etc.
A key thing for me recently has been to plan out what I intend to do over 10-15 sessions. I adjust my plan depending on how I am finding it. When I complete the planned number of sessions, I have a week off and feel pleased with myself. Then I start again, at a lower level, but not so low as the last time, planning to build up again to a bit higher than the last high point.
The key benefit to thinking about it like this, is that you can see a short term target. So if you miss a session, or have think you just had a too easy session, you don't have to stress because you know what your target is, and that you are getting there. If you find it is too hard, make the target easyer (you have to be realistic) and decide to meet your original target next cycle..
This is not in anyway revolutionary. But it has taken me years to work out how to train effectivly without going through the train loads get bored, stop, start again etc etc..
Hope that makes sense.
