Human beings remember "firsts"- the first time something happens, or the begining of an experience- and we tend to remember "lasts" as well. So when you are about to make a critical/negative delivery , start your criticism with a positive begining, it will affect the rest of the experience. Start by giving them solid ground to stand by expressing the fact that you value them and they matter. Once they are reassured of their own worth, people will accept your comments far more easily and they'll get less defensive.