I used sinew from kudzucraft. I also used Jeff Horton's lashing method, he wraps the end of the sinew around a dowel a few times and pulls on the dowel:
http://www.youtube.com/watch?v=Zy9JOv3mHgw
It makes it very gentle on the fingers. Perhaps it simply allows too much force to be applied? In his video he seems to be pulling quite hard, though. It's not that I have particularly strong hands (they mainly press keys on a computer all day...).