I have been doing this density training for chin-ups, recommended by coach Ethan Reeve and I must say it has pretty much worked like he said. This week I am up to 4 sets of 6 in 4 min. I could barely do 7 chin-ups when I started 2 months ago. I am pretty sure I could do at least 12 chin-ups in a row now, maybe more. I will find out next week!
http://www.charliefrancis.com/community/showthread.php?t=196&page=4&pp=15