I'd say fat intake. The higher your fat intake, the more your body can release natural oils to keep the hair nice and smooth and shiny. I don't use shampoo, though, there's a breakin time, so to speak, with not using shampoo, for your body to adjust, but now as long as I give my hair a decently good rinse, it's fine, no dandruff, etc. But, shampoo I feel is mostly a scam. You're washing the oils off your hair, then putting conditioner on to put oils back in, it's very pointless.
I had less fat intake for a week or two, and my hair was super sticky and nasty, then on days where fat intake was high (like 5-6 strips of bacon, bunch of eggs, etc) my hair was nicer. SFAs could help more, but even on days where I have a lot of olive oil, it's improved. Other things that help are the obvious, nutrition and vitamins, but stress is unfortunately still stress and still will have an effect. So far the best anti-stress thing I've found for natural stuff is astragalus, pretty much my favorite Chinese herb, it's full of choline so it repairs nerves.