Hydration is a big part too: I have venesections done every two week or so as a treatment for hemochromatosis. I.e. They take 450ml blood and dump it.

If my blood pressure is low before hand, the nurses get me to drink two jugs of water, blood pressure goes up to normal range with in minutes. Same before they let me drive home.

This has turned out to be a really useful tip for me life in general. If I'm feeling shit, I now suspect low blood pressure and force myself to drink two or three pints of water. It usually results in me feeling much better