Negotiating an offer, is it too late? - what worked for you?

I want to negotiate but I am afraid they will withdraw. The benefits are decent. What wording worked?

I thanked them and said I was excited but hoped we could talk about the pay. They came back with a bit more.

Thanks for writing this out. I went through something close to it 9 months ago, and it settled down once I kept a simple log of every call and form.