I would avoid Red Raspberry. Red raspberry leaf can actually bring on preterm labor for women with an irritable uterus, from what I've read + been advised. I was taking it with my first child & ended up having PPROM at 35 weeks.
I'm on nifedipine, but also take extra magnesium + calcium to help prevent contractions: 750mg of Magnesium, 1500mg of Calcium Citrate (most easily absorbed). Magnesium Sulfate is used in hospitals to stop preterm labor.
I also upped my protein intake on the recommendation from a midwife + a Chinese Medicine practitioner who specialized in women's issues. And I take extra vitamin C to prevent infection + 3 g of Alfalfa to prevent PROM.
And if you're allowed, they also recommended taking a bath with Epsom Salts (more magnesium sulfate). I'm not allowed to because I'm at risk for infection/PPROM, but if you don't have any restrictions you might want to try it.
I had a lot of contractions around 26 + 28 week, so would lie down on my left side, drink 60ml of water in an hour, plus 30 drops of Valerian Root to calm the contractions. Wild Yam is also supposed to help, but I haven't tried it. I also used the homeopathic Calms Forte to try & relax them + it seemed to work. Incidentally, you can also take it at night if you're having trouble sleeping.
Some midwives suggest that a small glass of white wine or vodka can help calm things down, too. Yes, I know it sounds crazy after avoiding alcohol for so long, but it did seem to help me. This is NOT an excuse to drink, and should just be used as an occasional emergency backup to prevent having to go to L&D with building contractions. Avoid red wine - it has tannins that can irritate women prone to preterm labor.
There's a great post at Mothering.com that gives a lot more info about ways to prevent preterm labor naturally:
http://www.mothering.com/community/t/385821/signs-of-preterm-labor. Just scroll down a bit past the info on how to tell if it's braxton hicks or contractions.
I've also been proscribed a Chinese Herbal medicine called Shou Tai Wan, or Fetus Longevity Formula. It's expensive (~$130/month for capsules) + you'd need to have a prescription for it, but I swear it's a miracle formula. I started taking it a few weeks after going to L&D at 25w for contractions. Was already on nifedipine + still having contractions. Started taking this, and I stopped having contractions, my cervix stabilized, and I have so much more energy again. I'm now at 31w4d & holding strong.
Good luck! Hope this helps.