Do bring lots of change with you
When she wants something she's going to want it NOW! Whether it's a cup of tea or a chocolate bar, you need to have change for the hospital vending machines. Running around with a £20 note asking nurses for pound coins will not go down well.
Do exactly as you are told
This is no time for a disagreement about the correct way to massage her shoulders. If she asks for something, do it, if the hospital staff ask you to do something, do it. Even if she asks you to go out for a bit and leave her alone, do it. And do it calmly and quickly.
Avoid pointless conversation about the view from the labour room window or last night's football (unless told to, of course). Yes, you're feeling nervous and gibbering about anything to avoid the subject at hand - the fact that a baby is going to come out - but she's the one having it, and needs you to be calm and supportive, not a rambling idiot.
Do speak softly and be reassuring at all times
Hold her hand, stroke her arm, mop her forehead and try to look at her face rather than what's going on at the other end. There are plenty of other people concentrating on that bit - a look into her eyes will calm her down. Tell her everything is going to be OK.
No shouting encouragement at the top of your voice. You are not Sir Alex Ferguson on the touchline.
Don't get in the way
Make sure you don't hinder the doctors, nurses and midwives. They know what they're doing and will keep you involved, but may not have time to explain everything to you as a running commentary. Remember when the boss's kid came to work and spent the whole day asking you "What are you doing now? What's that button for? Who was that on the phone? Why are you doing that?" Annoying, isn't it?
Don't keep nipping off to check your mobile messages
They can wait.
Do pack a bag of your own
You might have a lot of hanging around to do so bring a magazine, book, snacks, games console or anything else to keep you occupied during long waits between any action. Bring a toothbrush too. You might be there for a long time and brushing your teeth will help you freshen up.