I haven't been a birthing partner but my friend is being mine and these are a few things we've discussed and might be useful though you may have already covered them;
Beaware of not talking too much and please ask medical staff to shut up if they are talking too much about unrelated things
ie their lunch (several friends have had this issue - one was being whinged at by the midwife for taking too long to push and the midwife was missing her lunch break
and another friend said that having the midwife yapping the whole time was really annoying esp when she was trying to rest between contractions - unfortunatly she hadn't thought of this first and didn't want to be rude and tell her herself so now tells everyone to ask their birthing partners to be prepared to tell people to shut up :)
Try not to ask her too many questions, as again some women find it distracting. (You know your friend inside out and will know when or if this applies)
Know her birth plan inside out. That way if there are any problems or pushy midwives you can easily tell them what she wants without debate.
(For Example: I have put that I am not to be offered any pain relief, I will request it as I am sure I will be able to articulate wanting meds more easily than having to pay attention to refuse them)
Offer drinks regularlly with a straw - just hold it to her if she wants it she'll take a sip if not she'll just ignore you or wave it away.
During birth some women find that smells become really strong so I have asked that my friend doesn't wear any perfume / body sprays or eat anything strong ie garlic or curry if poss
My friend has packed a bag for herself as well - food and drink for herself, a toothbrush and paste, a few clean clothes (incase she gets anything on her ie my waters break over her or I throw up on her etc or its a long labour), swim wear (as I am planning a water birth and may want her to join me in the pool), cash for car parking and food (if I can manage to let her go to the canteen for food), her camcorder fully charged (I don't have one and may want things filmed as well as taking photos on my camera), take a note pad - that way you can always note down a few special details which might otherwise be forgotten afterwards.
Enjoy being part of this amazing experience :)