I really don't think there is anything at all you can do to cause or prevent morning sickness. Prenatal vitamins obviously don't make a difference for me, because I had been taking them for months/years before both pregnancies & I still puked up my guts for months. Do take the prenatals while you are trying to conceive, though...here is an exerpt from marchofdimes.com explaining why...
"It is especially important for women who can become pregnant to get enough folic acid. Folic acid, a B vitamin, helps prevent birth defects of the brain and spinal cord when taken before and very early in pregnancy."
But back to morning sickness...I had also heard that it could be due to the surge of progesterone early in pregnancy, but I argue that theory, as well. I miscarried twice, which was due to very low progesterone levels keeping my body from being able to support the pregnancy. But even with those pregnancies, with super low progesterone, I had morning sickness before I lost them. So with my two successful pregnancies, I did take progesterone supplements & had horrible morning sickness...but I only took the supplements until week 9, and had extreme nausea and vomiting until week 20-24.
It was better with my second successful pregnancy, though. With my son, I threw up constantly until month 5, then spent the rest of my pregnancy feeling like I would throw up any minute. I lost 20 pounds the first 4 months of pregnancy, was hospitalized twice, and spent quite some time on a home IV. But with my last pregnancy, I was nauseated for only three months, threw up quite a bit, but not nearly like before, only lost 5 pounds in the beginning, then started gaining in my 3rd month, and spent the last 5 months of pregnancy feeling pretty darn good. So there is hope that the second time around will be better. But you just never know. From my experience and the experience of those I know who have had terrible morning sickness, there is no rhyme or reason to it. Just stay hydrated as much as possible.
Good luck with conceiving, and good luck with your pregnancy.
C.