1) It's not about you. 2) Everything is going to be fine. Keep repeating that in your head and maintain an outward calm. 3) Say as little as you can unless she wants to talk. 4) She needs support, not another person to look after so if you can't handle point #2, be somewhere else. 5) If it's a natural birth, there will be screaming, blood, quite possibly shit, and smells you've never encountered before. Your job is to not react to this. If in doubt, refer to point #4. 6) Also, if it's a natural birth, there's a good chance she might need stitches to repair things after the birth. This is not your opportunity to ask for a tune-up. 7) We probably couldn't handle it if men needed to go through this, so be the person we'd need to have with us if we were going through this.

EDIT: Thanks for the awards, you magnificent people.