Give people sincere praise before and after any negative feedback.
"Sandwich every bit of criticism between two heavy layers of praise." One well known strategy for feedback is the “criticism sandwich,” popularized by the above quote from cosmetics maven Mary Kay Ash. In the sandwich, you begin with praise, address the problem, and follow up with more praise. In fact, the more of the conversation you can frame positively, the more likely your recipient is to be in the right frame of mind to make the change you’re looking for.
How to Give and Receive Feedback at Work: The Psychology of Criticism - Buffer