Know her birthplan, but try to take notice of when she needs to veer from it and support her rather than saying "but you don't want to do that". I had decided 'no epidural if at all possible' but got to the point where I was totally exhausted and needed to rest. I shouted that I wanted an epidural. DH held my hand, and said "you did say you didn't want one, HQ. are you sure?", but when I said "I really do", he realised I wasn't lying and didn't push the birth plan anymore.
Be prepared that you might see more than you bargained for, and try to take it stoically. The midwife might need you to stand near the action to help your DW brace against something (as my DH had to) and you will see everything. Don't say no, or whine, or faint! Focus on your DW rather than your own squeamishness.
Be on hand with water etc. My DH was a bit rubbish at this TBH - once I had the epidural he sort of sat back and read the papers while we waited but he was a bit rubbish at noticing if I needed water, or my hair tying back.
If your DW has long hair, learn how to tie it back into a ponytail and get all the hair off her face and back of her neck. You will be doing this about 20 times an hour!
If emergency things happen, try to stay calm. I lost a lot of blood and DH was left holding the baby while they sorted me out. He was very good at just talking calmly to me and hugging DS. Must have been more terrifying for him than me (I couldn't see or feel anything, he had a full view of all the blood pouring out of me) but you wouldn't have noticed.
Have a camera at the ready. The first photo of your DW with her baby is one she will cherish however bad she looks. Mine is on my profile page