Express your desired outcome. For example, instead of saying you want improved customer relations, state behaviors that indicate this, such as phone calls answered within three rings, friendly greetings given to customers or prompt responses given to customer requests. Be prepared to explain to the employee how your objective relates to the mission. People are much more likely to buy into an action if they understand its relevance and importance. Be assertive rather than accusatory.
Accusations distract the discussion away from the goal of the counseling session.
Future action is what you should be focused on, not past failures. Don't compare one individual with another. This alienates the employee. Instead, give specifics on how to improve the work. Be aware of obstacles.