I donated a lot when my father died. Just make sure you're not looking at the bag filling with your blood. We're programmed to think that our life is dripping out through that cord but be positive and aware that this helps someone and also regenerates your blood so it has its benefits.

Don't look and prepare mentally that you're not going to die there. I know it sounds absurd but the "fight or flight" situation kicks in and, like with panic attacks, the fear of actually dying probably leads you to pass out probably.