A good way to thicken your hair is to cut the hair to shoulder length or slightly above the shoulder.
And then also use some hair thickening shampoo and conditioner when you wash your hair.
Using the hair thickening shampoo and conditioner every few days when washing your hair can help the hair thicken out.
This has worked for me when I wanted thicker hair.
I've also applied some hair thickening creams to my hair at least once per week to help keep the hair thickened and it also keeps the hair healthy as well.
Doing a hot oil hair treatment can also help thicken your hair naturally and keep your shiny and healthy as well.
Another thing you should do to keep your hair thickened and to thicken your hair is to use some volumizing styling products.