Work out the way to the hospital and don't dawdle along on the way there (my mum still holds a grudge against my deceased-for-twenty-years grandad, who refused to drive over 20 as it was dark, whilst she laboured in the back of the car. She hasn't forgiven my dad for not checking that their own car was roadworthy before she went into labour)
Familiarise yourself with any technology e.g. Tens machine. Don't show off to the other birth partner and electrocute your wife with it by accident as your son is crowning.
Be confident, calm and reassuring even if your wife is screaming she can't do it any more and would like everyone to fuck the fuck off to the far side of fucking fuck.
Make sure that food, water and flannels are available at all times.
Tell your wife she is a freaking goddess after the birth. Look into her bloodshot eyes and tell her she has never looked more beautiful and you have never loved her more. Whenever anyone asks about the birth tell them that Mrs Grooves was absolutely fantastic and you are in total awe. Never mention a)the shit, b)the blood or c)anything stupid she might have said under the influence of narcotics.
Read all the books you can so you don't have to ask loads of stupid questions when your wife is in the middle of a rhinoceros through a postbox experience.
Hide some of her favourite, most craved food for afterwards. When I was cradling new DD, DH brought in the smelliest blue cheese ever (the rest of the ward hated me) as well as plenty of fresh fruit adn crackers.
Don't try and fix her pain or negate it in any way. When I was struggling with a long labour and began to cry, DH didn't offer me anything, he just took me in his arms and held me and that was when I realised that I'm glad he is my DH