Wool pillow case
Wool helps, but like Eagle said it'll cause tons of frizzies. Plus it's really warm in the summer. If you want, you could just get a normal cotton pillow case, but something like 200 count thread. That'll be comfy, and rough enough that it'll cause some knotting without making your head into a puff ball.
I started mine 5 years ago not knowing about this site (if it's even been around that long). I just went with my gut, which happened to lead me to do practically nothing. And 5 years later I couldn't be happier. Of course there are things you can do to speed up the process, but none of it will help you grow locks any better than another. Locks will grow no matter what you do to them