I go through periods, such as now, where I do "back to back" braiding in cornrows. One reason I think I don't experience damage and I achieve massive hair growth is because I braid them myself and I DON'T pull them tight, so as to break my hair. Becauuse of my hair texture, I simply put on a creamy conditioning leave in on each sectins before adding extentions and breading it. I do put beads on my hair and always have, however, at night, I have to use 2 satin scarves to tie my hair up. The first, I tie on my head, leaving the braids hanging out the back. Then, I wrapt the beads in the second scarf, and then slightly lift it and securly tie it to my head too. I guess I should take pics of this and post it on my site so you can see what I mean. But, by doing this, I don't stress my hair and it grows really well.
Also, I tend to take my braids out every 2 weeks, and when I do, I put on hot oil and sit under the dryer, then wash it with CON, use a protein conditioner (without heat), and rinse it completely out. I then put in serum, followed by letting my hair dry until just slightly damp, to which I add biosilk. I let it thoroughly dry and I try to go one complete day without the braids or doing anything to my hair. Once I braid my hair, if I feel any pulling, I take it down and redo it. Because I use large beads and "washers" I have to make sure when they're added I still don't feel any stress on my scalp. I plan to continue braiding my hair until summer of next year...then I'll probably relax it again and take more pics so you can see the amount of growth.
BTW, I have seen people get braids back to back and have their hair thin, so I know it's possible. I just happen to be one of those people who doesn't experience this...not yet (she says as she knocks on wood)