I agree with rc. This conundrum comes up quite often and the advice is consistent.1. Get the second job offer in writing first2. Write a professional letter/e-mail to the first company advising them that you will not be joining due to an improved offer from another firm. Be as low key as possible and don't get dragged into a discussion on counter-offers etc.You will be burning your bridges with the company that you are now turning down, and the recruitment firm if there is one, but frankly, who cares. As rc says, they'll forget you in a month.