What's the reason of preterm labor and how to prevent that in next pregnancy?

Well many reasons. To avoid a preterm labor depends on the cause. So with the next pregnancy see doctor right away and follow. Lovely with the doctor. .
Good prenatal care. Is the key, causes for preterm labor are numerous, cannot be summarized in 400 characters, the best way to avoid, is to have good obstetric care and rigorous prenatal follow up given your history, wish you well .