For the record, I'm not a cosmetologist. However, I've spent a fair amount of time working in salons and obsessing over my own hair. Along the way, I've accumulated some knowledge...
+ Drink lots of water. I think it's one of the most important things when it comes to maintaining a healthy body, and a healthy body will produce more healthy hair.
+ Your diet plays a big part in hair growth and health. Our hair is made up of a type of protein called keratin. I like to think of it as excess protein because our body uses what it needs and then gives the rest to our hair, nails and skin. A protein rich diet is a huge factor in hair growth. Vitamin D, vitamin E, omega-3 fatty acids and biotin are also great for long locks. You can find protein, vitamin D and omega-3s in salmon. Walnuts have vitamin E and are a natural source of biotin.
+ Biotin supplements! You can find them at almost any grocery store and they're not expensive. I'm not sure how exactly biotin supports hair growth, but lots of research shows it does. and I'm willing to trust the scientists on this one. I took the supplement daily and I really think it was a big contributor to my hair growth.
+ Switch to sulfate free products. Quality hair products make a big difference in the health of your hair. Personally, I love Aveda products. I've heard great things about their Invati line, but haven't tried it out yet.
+ Be aware of how you brush your hair. Ripping through tangles is a definite way to break off ends of hair. Always (calmly and slowly) work from the bottom up. Use a wide tooth comb on wet hair.
+ Eliminate as much heat as possible. I only wash my hair 1-3 times a week in order to keep the blow drying to a minimum. When I do blow dry my hair, I first let my ends air dry almost completely. I also use the curling iron and flat iron as little as possible. This one is a biggie. Heat really damages your hair and causes breakage. When you do heat style, be sure to use some sort of thermal protectant.
+ Keep the bleaching at bay. Like heat styling, bleaching your hair is very damaging and will cause breakage. While growing out my hair, I wore my natural color with a few facial framing highlights to break it up a bit. Coloring your hair isn't as brutal as bleaching, but still does cause some damage so avoid it as much as your can.
+ TRIMS. TRIMS. TRIMS. Trust me, I know cutting the bottom of the hair that you've worked so hard to grow every 6-8 weeks seems insane, but it will help you grow long and healthy hair. Ends that go untrimmed will split and eventually break off. Trimming your hair keeps it pretty. What's the point of having long hair if it's broken and ratty at the ends?
+ Give into the conditioning treatments offered by your stylist once in a while. Treat Yo'self
+ Rub your head, or, if you're lucky, have someone else do it for you. Scalp massage increases blood flow to the hair follicles and promotes growth. Use the pads of your fingers to massage in slow circular motions. Make this part of your daily routine (I do it while watching TV before bed).
+ Have patience. Hair doesn't grow overnight. In fact, hair grows in three stages: Anagen (active stage), Catagen (transitional stage), Telagen (resting stage). Everyone's hair goes through these stages at different rates.
It's probably not necessary to follow this list religiously, but knowing how to properly take care of your hair will ultimately help it grow. My hair hovered at my shoulders literally for years until I started practicing proper hair care... and now I'm hair obsessed.
Good luck and happy growing!