What is the best way to go about a salary negotiation?

I certainly don't want to sound harsh, but getting a 25% increase in today's job market is pretty damn good. I think you should just take what they offered you and show them they did not make a mistake by putting you in the position. If you were to ask for more I don't think they would retract the offer but they might ask themselves if this person is just in this for the money or are they committed to the company and grateful for the opportunity for advancement and a career.