Active listening. It's tempting to interrupt and plan what you want to say instead of really hearing what the customer wants. You have to paraphrase what they are saying and ask important questions if you don't understand. 

Listening skills are becoming so rare today. We're all so distracted by our smartphones and issues in both our personal and professional lives. I'm one of the most guilty. But I try hard to practice listening skills because customers and even friends really appreciate them!