Treatment to Thicken Hair?

Several medications can improve the thickness of your hair, including oral finasteride (Propecia, for men) and topical minoxidil (Rogaine). They are both safe and very effective in increasing the caliber of your hair and increasing the time it spends in the actively growing phase. Just remember both require 6-8 months to start to appear full. Don't give up after a few weeks!
Some other products can be applied directly to the scalp to help increase the appearance of density. Toppik, X-fusion, and Joan Rivers' "Great Hair Day" can all be applied as a camouflage to the scalp to increase the look of fullness. Just remember that they won't GROW hair, just change how it looks.

And, if you are still not happy you may want to consider hair transplantation. If you are not on blood thinners, don't smoke, and have sufficient density in the back of the scalp, you may benefit from moving those hairs to the front where the density is thinner. Contemporary hair transplantation can offer natural results which are impossible to detect!

Tthere are certainly things you can do to not "damage" your hair and those things are avoid harsh chemicals, avoid the chemicals that are often found in coloring agents, treat your hair gently and avoid traction ie pulling on the hair shafts by pulling the hair back, used soft gentle approach to brushing with wide tooth combs, do not over wash, do not over condition and do not over treat with hair products, sprays, and gels.

From a medical perspective, if you are concerned with a possible hair problem, thyroid and some dietary abnormalities can predispose to hair problems. So, if you are concerned that a medical explanation might account for hair abnormalities, I would suggest following up with a dermatologist.