Try deep treatments first. Simple ones work great..like olive oil, pure coconut oil , avocados, honey and stuff. Also try a protein treatment as straightening strips away protein from hair.
First make sure your hair is properly moisturized. Leave in conditioners make sure that the moisture is retained in hair but it does not provide the "real" moisture that heat/chemically damaged hair needs.

If I were u, i'd deep condition on clarified hair twice a week if I had the time. If u want to increase thickness of your hair you can use cold pressed castor oil
HTH

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HAIR TYPE -2a-b, black, mostly medium
CO WASH, RO-HE HH and ENLEVEN and GARNIER RANGE
CLARIFY- I use sulphate shampoos silicone free(diluted), vinegar rinses
DC-pure coconut oil, olive oil, sometimes a SMT
LI-aloe vera gel + castor oil
I'm currently at BSL, aiming for waist length.I'm a very impatient person, wish me luck
I love my flat iron, but I love my waves even more. :-D