Rosemary and clove water and keep your hair out of twists and braids.
My brother had a thinning area on his crown, but since he stopped twisting it, braiding it, and bunning it, as well as using the rosemary clove water, it's thickened up considerably.
Also, if you're not Muslim, vegetarian or vegan you can try taking gelatine. I take it for other reasons, but it's thickened my hair and nails.
People of our ethnicity tend to have fine to regular hair, meaning the diameter of our strands are not that big as opposed to people of Asian decent who have coarser hair. So, we have to make sure we're maintaining practices that nourish our bodies inside and out.