I use the sea bands. Swear by them. No side effects, either.
But, like all motion sickness treatment, you must use it before you start to feel nauseous, or it likely won't work too well, if at all.
If you do feel seasick (and i've seen people ill while sailing on mill-pond conditions - everyone is different), the best thing to do is to throw up; you'll feel a lot better almost immediately.
Some say lying down helps; it's certainly better than moving around the ship if it is rocking around.
Fresh air is good, too, looking at the horizon. But if the ship's bouncing around, you're not likely to be out on deck anyway - dangerous.
Crossing the North Atlantic in a force 12 gale, the purser suggested (suggested!) that passengers not go out on deck. Well if anyone did, they'd have been washed overboard just like that.
The charming girl who served us supper I remember looked quite green and I felt sorry for her having to move about and look at all that food while feeling so wretched.
Gulp....I've heard some things about being on the "Spa" deck that have concerned me. Am I looking at getting sea sick because of where our cabin is ? My husband and grandson almost did on the last cruise we were on and we weren't that high up.