1) Put on your crocodile skin.
That precious prose that you struggled over weeks and months to get perfect is going to be attacked by a person with a red pen and a machete; your editor. They will begin to cut out sections that do not forward the plot or are unnecessary detail. Don't take offence at this, that is why you brought them onto your project after all. They have done a lot of books in all likelihood so use their experience to your benefit. Listen to their comments.
2) Don't take the edit personally.
Try to be objective. If the editor says that a passage has an issue, look at the their note then read the manuscript with that in mind. Look at it as if you were a reader. If there is merit in their observation, you can decide to accept it. If you feel they are making a mistake, like deleting something necessary to the plot, you can push back and state your case clearly.
3) Be professional.
You expect to be treated with courtesy and respect, yes? You get what you give. The editor is there to make your novel better. Better = More sales. Keep that in mind. Flipping them off verbally or an endless stream of argumentative emails back and forth accomplishes nothing. Take a step back from the issue and address it calmly. It is a collaborative process and both of you need to be involved.
4) If you get mad...
I know I did a few times. They just cut what you feel is a key piece of text. OMG, how could they do that to me? When that happens, take a break. Walk away for a while. Go for a swim, play with the dog or do something else relaxing for a day or two until you calm down. Then go back later and reassess the situation objectively.
Do you have any other tips from your experiences? Feel free to comment.