The only thing you really can do is make sure your drinking enough fluids. I have heard of women who are dehydrated going into premature labor. I think low fluid is just one of those things that happens...just make sure you are drinking enough water, and are drinking more than recomended on hot days or days you are active.
I had a doctors appt yesterday and found out that I am dehydrated. He said that is one of the biggest reasons for preterm labor. He said that I needed to be drinking FOUR liters of water a day