Am guessing from your question that you're not looking for something to actually thicken your hair, ie grow more, but something to give the cosmetic effect of thicker hair.
I've been thinning in the crown for the past couple of years - quite noticeably over the last 6 months or so. Alongside treatments, just got some toppik the other day - it works ridiculously well at filling in thinning on the crown. I know its not a "cure", but it's totally filled in my thinning patch and, even with a couple of powerful mirrors, it's very hard to detect... Good for day to day confidence, and keeps the "haha you're going bald" jokes at bay, which is always appreciated!
Good luck....