R.J.
Probably nothing. Hair alleles are one of the more complicated bits of genetics. People from the same family can have radically different hair from each other depending on how their DNA tells hair to grow (says the only corkscrew ringlet curls in my own family of over 40 people). No matter how much I may WANT my sister's or dad's or uncle's hair... it's just not going to happen. I've had to learn to deal with what I've got.
That said... the single most common cause for hair problems is malnutrition (including dieting). Hair is 100% protein bound together with hydrogen bonds. If your diet is lacking in protein your body will skimp your hair and nails before it starts eating your own muscles. Vitamins and minerals help the follicles and oil production in your scalp, which can help hair's overall health, but protein is what the hair shaft is made out of. If you don't have enough protein in your diet, your hair will suffer.
The best thing I ever did as far as my own hair is concerned is to link up with an awesome stylist who really helped me with making MY hair the best it could be... instead of how I wanted it to be. I've also learned how to relax it, blow it out, and in other ways deal with it in a way that makes me happy. It'll never be the hair I want, but it's the hair I've got... so learning to make the best of it has been a big blessing.