Disclaimer:
Obviously only to be used in addition to, and never in contradiction of your doctor/mid-wife's recommendations.
Nothing being suggested is known to be harmful to you or baby in anyway. If it can't hurt, it's at least worth a try.
1. Drink lots of water. Seriously more than you think you can drink. 96 oz per day or more. Around 4 litres - 1/2 water (so 2 quarts) and 1/2 other liquids (such as milk, orange juice, decaf/herbal teas)
2. Along with this it should be noted to NOT restrict your salt intake. Sodium makes you retain fluids (not always good) but in your case, retaining your fluids is a good thing. Iodized table salt is not very good for you. If possible use UNREFINED sea salt (usually slightly moist and gray in color).
3. Make sure you are getting LOTS of protein. 100 grams per day in general for a healthy pregnancy and to help prevent preterm labor. If/once preterm labor symptoms occur 150-200 grams of protein per day. For every hour you are awake, eat something that contains at least 10 grams of protein. Eggs are excellent. Eat at least 2 or 3 everyday. Eggs also contain choline. A choline is sometimes recommended to help preterm labor, but if you eat a few eggs that should be enough.
4. Omega 3 Fish oil supplement. The best kinds are krill oil, cod liver oil or salmon oil. Good for lots of things in addition to preterm labor like brain and eye development. Very good for mom and baby in general. Believed that the particular form of vitamin E in fish oil can help inhibit the formation of prostaglandins (that effect the cervix) and also reduce cramping.
5. Check to make sure you don't have a undiagnosed infection of any kind (kidney, UTI, lady parts, even gum disease).
6. Probiotic supplement (just eating yogurt is probably not enough). Good for mom and baby in general. Can help treat an imbalance of bacteria in digestive tract and in lady parts. Imbalance of good/bad bacteria can sometimes be enough to irritate the uterus causing contractions. Probiotics in pregnancy and while breastfeeding have been proven to lessen digestive issues, allergies, and eczema in the baby and also prevent thrush and other infections.
7. A liquid calcium/magnesium supplement (in addition to your regular prenatal vitamin). Sometimes the ingredients used in large-scale produced vitamins don't get absorbed well by your system and you end up lacking some, particularly calcium - which may contribute to cramping. Get plenty of dairy since it is a good source of calcium and also protein. Don't eat/drink non/low/reduced fat stuff. Go for the real/whole stuff. Whole milk, yogurt, cheese, and butter.
8. Lack of vitamin C is sometimes associated with premature rupture of membranes, so drink some orange juice.
9. Yoga, relaxation exercises, deep breathing, meditation/prayer have all been shown to help. Also, whatever you can do to reduce stress of any kind.
Take warm baths or even go swimming if the doc/mid-wife says that's ok.
