How can you tell when a Ball Python is going to bite?

[nybaby]

This is my first time owning a Ball Python. I had him for about 3 weeks. I only held him maybe twice for only about a minute before I got scared and had to laid him back down in his cage. I don't have a snake phobia or anything and I'm not afraid of snakes. I'm just afraid of getting bit. Some people have said its inevitable to not get bitten when owning a snake. Others have said that they've owned tons of snakes and they were never bitten. I know a snake only bites when threatened. So how do I pick up and hold my snake without him feeling threatened? What scares me the most is when I pick him up and he slithers in my hand then starts to stare at my fingers and gives them the same look he does when its feeding time. I get so nervous my hands start to shaking. Maybe I'm just overacting but I just don't want to get bit.