Its quite simple really.
deep conditioning has been a big factor in my thicker hair.it intensely moisturises your hair and keeps it strong and flexible.the more you do it the more thickness you will see.
i currently deep condition with steam 1-2 times a week but once a week is adequate.do what your hair tells you if you need more moisture then do more deep conditioning treatments.
you also need to balance the moisturising deep conditioning treatment with protein treatment.
the rule of thumb is heavy protein treatment every 2 months and light weight protein treatment every 2-3 weeks.
minimum heat is essential if you want thicker healthier hair so air dry your hair more often and use heat protection if you need to use heat.
if you relax your hair try to stretch your relaxer to at least 12 weeks this will significantly reduce relaxer ovelapping.