I never get a headache from it, but for the nausea -- fresh ginger root.
I've always had stomach sensitivity with kava. Recently I read about, and tried, fresh ginger root. Here's what I do:
I slice off a chunk of the root, maybe an inch's worth. Then I peel it, and slice it into as many slices as shells I intend to
consume, plus 1 -- 4 shells, 5 slices. Each slice should be 1/8 to 1/4 inch thick, depending on how big the root piece is.
I eat the first slice before I start brewing the kava. Then, I eat a slice after skulling each shell.
I no longer have ANY nausea at all. It really works!!
Also, make sure you drink enough water, as kava is a diuretic.