This is one of the ugliest issues we have to deal with in our profession. I have worked both as an executive recruiter and on the inside as a VP of Human Resources. Having been on both sides of the desk there are few things more damaging than loosing a candidate to a counter offer. Without going through all of the issues i.e. (lost time, creatability, money, etc.) I tackle this issue right up front. One of the first things I tell my candidate to do if they indicate any interest in entertaing the counter offer is go in and tell their boss they have an new opportunity and they plan to except the position. Most baulk at this but I explain to them that there are a limited number of issues that their boss can change and if there only reason for making a move are because of their unhappiness with certain issues why not see if we can get these issues changed up front. This will help begin the closing process with our candidate right then and there. The ideal candidate realizes why they want to make a change, is it because they are unhappy or are they truly looking for a better opportunity. If they are unhappy this will give you very solid information about the reasons for the disatifactions and allow you to evaluate the viability of your candidate, and are his/her reasons that their boss can potentially change if so red flag. There are to many variables to go into here but most of us know what to look for. Another technique I use is to role play and put them into the process to the point that the company has made them and offer and they have accepted it, resigned their position, put the house on the market, prepared the kids to transistion to a new school ect... and have the company come back and say you know we've changed our mind and we really don't need you. How would you feel? Well that's essentially what your doing by taking a counter offer. If you can't get a guarantee from your candidate at that point, probably best to move on to the next candidate.