I'm a type 3b, and I currently have hair that reaches just below my boob(when stretched)I had that same problem, and three..no, four things helped me.(At the time I started following these I was right around collarbone length(straight).
So, here's what helped me.

1)Obviously I gave up heat. Simple start, obviously.

2) I stopped getting my hair cut regularly. You know the whole, you're supposed to get it trimmed every 6 weeks thing? Yeah, I was cutting off all my growth.Now I don't trim it unless I actually see it's damaged badly and needs to be trimmed. That helped alot.

3)As far as stretching it, I braid it while it's damp, anywhere from 1 to 4 braids depending on how wavy/straight. I want it to be.

4) Treat your hair like lace. SUPER DELICATE. Don't let it get stuck under things, don't brush it, no heat of course, no teasing, no nothing.

5) The ultimate solution. It depends how much you'll do for long hair fast. This is what I do for a 1/2 inch of growth of week-Miconazole Nitrate. Now before, I'd have neverrrr used this, simply because-it's a cream for vaginal infections. But, after reading soooo many threads with so much success from the Long Hair Community and other places, I had to try it. so I did, and I've had so much success. Apparently it kills growth-inhibiting bacteria on your scalp, thus allowing your hair to grow more. I've had so much success, just massage into my scalp everyday. I really would look it up if you're serious about growing your hair long.

I hope any of this helped!