Listen. Make your wife understand that you're there to listen. If you think that she's overreacting about the changes in her appearance or that she's worrying over nothing, just keep those thoughts to yourself. Listen to her and help her go through it.
Pack. Ask your Ob-gyne about the things you need to prepare. Pack them all in your hospital bag. It is best for both of you to be prepared all the time. Your baby might come out earlier than expected. Ask your wife what else she needs to bring with you. Do the packing while she does the talking. :)
Have an assistant. You might forget a lot of things no matter how prepared you are. Have someone assist you on the D-day. Let your assistant contact your relatives and carry all the things you need to bring. If possible, leave the driving to your assistant as well. This will allow you to give your full support to your wife.
Don't be ignorant. Read books and stay informed. Go to your OB-gyne together. Be proactive. Ask your OB-gyne the stages you're in and the things you must do to ensure everything is okay. This will keep you from getting startled when the D-day arrives.