To keep hair strong and healthy, take a daily multi-vitamin. Your hair is basically dead cells and cells need nutrients to be healthy. I also take biotin, which is a really good supplement for hair and nails. But I am not a doctor so the only thing I will be recommending is a daily vitamin because everyone should take one.
This is the daily vitamin I take.
I also thing they make my hair grow a little bit faster, but I have no idea.