This is a great review from the British Medical Journal of tocolytic therapy, treatment to stop preterm labor. It reviews  what works and what does not. There are two tocolytics that seem to have the highest success rate: prostaglandin inhibitors (eg indomethacin, ketorolac)  performed best closely followed by calcium channel blockers (nifedipine, nitrates).

"Tocolytic therapy to delay preterm delivery is an important intervention in obstetrics. Although tocolytics have not been shown to improve neonatal outcomes, they can delay preterm delivery long enough for antenatal corticosteroids to be administered or for the mother to be transported to a tertiary care facility. In premature neonates, antenatal corticosteroids reduce morbidity and mortality. Tocolytic therapy may therefore have an important role in improving outcomes from preterm delivery. With over 500 000 preterm births in the United States alone (12.3% of all births in 2008)3 and 29% of these being less than 34 weeks’ gestation, preterm delivery is an important public health issue."

"Many different classes of drugs have been used for tocolytic therapy. These include beta mimetics such as ritodrine and terbutaline; magnesium sulfate; prostaglandin inhibitors (for example, indomethacin, ketorolac); calcium channel blockers such as nifedipine; nitrates (for example, nitroglycerine); oxytocin receptor blockers (for example, atosiban), and others. Each tocolytic has a unique mechanism of action, side effects, and degree of complexity to administer."

"Balancing the results relating to benefits and harms, this systematic review and network meta-analysis on trials of tocolytics found that prostaglandin inhibitors and calcium channel blockers have the highest probability of being the best therapy for preterm delivery on the basis of the four outcomes: delivery delayed by 48 hours, neonatal mortality, neonatal respiratory distress syndrome, and maternal side effects (all causes)."

"Of all the classes considered, prostaglandin inhibitors had the highest probability of being the most effective class for delaying preterm delivery and had the most favorable maternal side effect profile. They did not, however, perform as well for the neonatal outcomes. When the probability of being ranked in the top three treatments for delaying delivery was considered, calcium channel blockers also performed reasonably well, with a 57% probability of being the best class. For the two neonatal outcomes, calcium channel blockers also had the highest probability of being the best class."

Read more in the British Medical Journal