The better solution to arrive in Montepulciano is to use the italian Highway A1. You must search for the exit Bettole-Val di Chiana , and you take the direction for Torrita of Siena and after exit from Torrita, on the left, you must follow the Montepulciano indications( only 15-20 minutes).
Also, If you come from Siena, you must drive for Siena-Bettolle with A1 direction and you must exit to Sinalunga. You must continued in Torrita direction and after for Montepulciano.