How to negotiate an offer

Hi all, I have received an offer from Accenture which is on the lower side on the market standard and less than what i was expecting. I know they can raise the offer but I don't know how to approach it correctly ,to ask them without offending or you know being rude or showing a negative side. They are giving excuses like my current salary and market and they can't raise it etc etc. I have one round of discussion with them and i told them I will let them know .