A few things. We don't really know what causes preterm labor, but there are a few things that tend to increase the risk: serious trauma (wear your seatbelt!) smoking doing drugs high order multiple pregnancies uncontrolled diabetes but for the most part we don't know. A great answer would win the nobel prize for medicine!
Live well. Keep going to and listening to your dr, eat well and healthy, live safely, wear seat belts, do not smoke or drink alcohol. By doing all these things, you have the best chances of carrying to term and having the peace of mind that you know you did the best job you could to have the healthiest pregnancy possible.