In essence, accepting a counteroffer is tantamount to career suicide. Statistically, the odds of benefiting from accepting a counteroffer are akin to the chances of being struck by lightning. You do not have to take our word on the subject. Plenty of research has been done and much has been written about it. By all means, exercise your due diligence and research the topic should you find yourself tempted by a counteroffer.
So, how do you respond? Just as you tendered your resignation, you respond with professionalism and grace. You politely tell your employer that your decision to resign is firm. You wish them well and look forward to the rewarding opportunity awaiting you with your new employer.