the two main things are good blood flow in the scalp and nutrition, especially zinc. reduced breakage and shedding/pulling also will increase the rate it grows... it's a flow process, the growing minus the loss.
i'm expecting my hair to thicken up a bit now i have stopped boozin :)
my mother always used to have issues with it. i suspect she had a mild gluten allergy that messed up her digestion. probably zinc is a slow to absorb mineral. stress also makes hair shed more often as well.
I take extra zinc every day (with copper because zinc interferes with copper absorption).
