What is the scriptural evidence that Mary was always a virgin?

Protestants and Catholics both share the belief in the virgin birth - that Jesus was conceived by the Holy Spirit. But many people think the scriptures are also clear scripture that Jesus had brothers, and that Joseph did not have sexual relations with Mary until after Jesus was born, meaning that they did have sex later on. Against these views, what is the Biblical Basis for the doctrine that Mary was a perpetual virgin. I'm happy to have answers from anyone, but the scriptures should be from the Protestant canon.