One of the main issues affect hair length is breakage. You can grow your hair out but if it's subject to poor care it will break more often, resulting in a shorter length.
The best way to care for your hair is to take good care of it, so it will grow longer! Trust me, nothing can really force it to grow faster-- but it can grow healthier and break less with proper care.
For a health issue, my doctor put me a full regimen of vitamins. After taking them for about a month and a half, I noticed that I was sprouting "baby hairs." My hair has been growing more, but not necessarily more in length. What I did was chopped off a lot a couple months ago, but with a better diet and hair care, it's thicker and it's growing longer. Also, refrain from those "monthly" trims that you always hear about. I can't understand how it's suggested to trim your hair once a month or even once every couple of months when the trim eradicates most of the hair growth. Yes, it's nice for getting rid of split ends but it's better to prevent the split ends in the first place.