Well, don't cut it, dye it, use harsh products, or irons on it. There's a vitamin called biotin that helps growth And If you flip your hair upside down and massage your scalp it helps open up your follicles! :) this has all helped my hair grow and I thought I'd never get it as long as it is!
Yeah. This sounds weird but try to be in a low-stress, healthy environment. I was at a bad old school and my hair barely grew 3 inches in a year and I switched to a better, more friendly school and it grew about 7 inches in 2 months.