Be _VERY_ careful of megadoses of vitamin B6. Especially if your current children have any evidence of epilepsy, a tendency toward febrile seizures, or if there is a family history of epilepsy on either side of the baby's family.
"Research has shown that infants of mothers who took high doses of vitamin B6 during pregnancy may be more likely to develop seizures." ...That's a quote from Dr. Sears' pregnancy book, and something I've heard from my doctor, too.
My kids have epilepsy, and it's something I wouldn't wish on anybody. I know morning sickness can be very, very serious, but a lifetime of seizures and anti-epileptic drugs is far more dire.
(Also note, high doses of vitamins A, D, and E have also been shown to increase the risk of birth defects including cleft palate and heart defects. High doses of vitamin C have been shown to cause vitamin dependency in infants.)
Enough with the scary stuff....I had awful morning sickness with the twins, and the acupressure bands (or Sea-bands) really did help. Another powerful help was pineapple juice. My mother used it to combat nausea when she was going through her chemotherapy, and for some strange reason it worked for me, too. You would think something so sweet and acidic would be nasty, but it's okay.
I always had a hard time getting enough fluids in the first trimester, and two things that helped were drinking water with large sqeezes of lemon and drinking iced peppermint tea:
put 1/2 to 3/4 cup of boiling water in a mug, and add a peppermint tea bag. Brew for 3-5 minutes. Fill mug with ice, wait for it to melt, and YAY! "instant" iced tea.
Food-wise, bland is the best. Plain graham crackers!! Anything with an aftertaste was too horrible. Watermelon, cantaloupe, Frozen "Whole Fruit" popsicles...all good ways to be gentle to your stomach. Drinking fruit "nectars" rather than fruit juices will help, too--it'll give you more fiber and more nutrients than juice, and the fiber certainly helps with constipation.
It sounds like you had a really rough time with your other pregnancies, so a lot of this is probably old hat to you.

. Sorry it's so rough. I hope this time is easier.