Franco,
Follow the link in Lynn's post
www.crossfit.com/cgi-bin/discus/show.cgi?tpc=21& post=7774#POST7774 to Ethan Reeve's description of Density Training.
As an example, if a total of 40 pull-ups is your goal, you could try the following progression. Try to start each set every minute on the minute. When that level gets easy, then move to the next.
Sets x Reps = Total.Reps (Time Goal in min.)
16 x 5 = 80 (16)
14 x 6 = 84 (14)
12 x 7 = 84 (12)
10 x 8 = 80 (10)
9 x 9 = 81 ( 9)
8 x 10 = 80 ( 8)
8 x 11 = 88 ( 8)
7 x 12 = 84 ( 7)
7 x 13 = 91 ( 7)
6 x 14 = 84 ( 6)
6 x 15 = 90 ( 6)
5 x 16 = 80 ( 5)
5 x 17 = 85 ( 5)
5 x 18 = 90 ( 5)
4 x 19 = 76 ( 4)
4 x 20 = 80 ( 4)
etc...