What can I do to prevent another premature birth?

MFM. You see a maternal-fetal-medicine specialist before you conceive. (mfm). You sit down and discuss what caused previous preterm births - was it a problem with cervix, was it infection, etc. Then together work up a plan to minimize risk. There are no guarantees, but you can decrease risk by seeing the right people before and planning ahead.
Progesterone. In women with a prior preterm birth, weekly Progesterone injections can be given to help prevent another preterm birth. Injections are started between 16-20 weeks and given every week until 36 weeks. It can decrease the incidence of preterm birth by about 30%. It is not given to women with twins or triplets.