How do you decide which parts of the bible to take literally? Obviously, we all don't handle snakes.
I look at the context and surrounding text.
The way I read it for the snake handling thing, Jesus is speaking to a specific group of people regarding what will happen when people become believers through their sharing of the gospel:
Mark 16:14-18Later Jesus appeared to the Eleven as they were eating; he rebuked them for their lack of faith and their stubborn refusal to believe those who had seen him after he had risen.
He said to them, "Go into all the world and preach the good news to all creation. Whoever believes and is baptized will be saved, but whoever does not believe will be condemned. And these signs will accompany those who believe: In my name they will drive out demons; they will speak in new tongues; they will pick up snakes with their hands; and when they drink deadly poison, it will not hurt them at all; they will place their hands on sick people, and they will get well."
I don't see this as a command that people in this day and age are to handle snakes in church.