Negotiating an offer, is it too late? - 8 weeks in

I received an offer as a data-entry clerk and I think the pay is a little low. I have been trying to stay organised about it. I have a competing interview next week. 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 14 months ago, and it got easier once I kept a simple log of every call and form.