Mr Groove
Don't sit playing on your phone the whole time.
Don't keep wandering in and out to give your DM or DMIL regular updates on the state of Groove's cervix by phone, facebook or text unless this has already been agreed in advance by Groove.
Don't ask questions like 'do you want a drink' once labour is getting going. Just offer the cup (with a straw in) at fairly regular intervals. Ditto things like glucose tablets if they're in the bag. If the G&A is being used, offer lipbalm regularly too.
Read the birth plan. Don't just 'go along' with what is suggested on the day if Groove has made it clear that's not something she's happy with (such as routine examinations, or an epidural) - ask questions on her behalf if she can't (like 'What happens if we wait another half hour?') or if a question is posed mid contraction, suggest waiting until the contraction is over before asking again.
Don't ask if it hurts. Yes, yes it fucking does. Lots. With bells on.
Familiarise yourself with the labour bag and birth bag so you know where things are.
Expect poo. From Groove, the baby and possibly yourself when you realise that this really is IT.
Expect to find yourself down at the 'business end' amazed at the fact that there's actually a baby coming out of there, even if you're very squeamish and have vowed to stay up top.
Don't panic if things don't go to plan. Wherever you are (home, MLU, CLU) the MWs are trained to deal with all sorts of emergencies.
Don't be surprised at how vigorous the MWs are with your baby once it is born - DH was
at the way the MW handled DD when dressing her (although it was considerably more effective and quicker than his dainty-fannying-about attempts)
Eat and drink if you can. Keep your own energy up. You'll be no use to man nor beast if you're feeling all faint from lack of food later on.
If Groove threatens you with serious harm, no sex ever again, divorce or anything else, at that time she means it. She won't mean it later. But she will at the time and God Help You if you don't stop whatever you're doing to make her threaten you!
Enjoy it - this is the exciting bit where you actually get to meet your baby!