What to do to Thick hair to make it thinner?

My hair is so thick, that if I flat iron it, it's just a tiny bit less poofy. It's UNBELIEVABLY THICK. and I've gotten it thinned out like 2-3 times, one just a couple weeks ago. So that's not an option either. Also, when I get a layered haircut my hair poofs like crazy. I'm just wondering what type of haircut to get? for sleek, smoother, thinner hair?