Prevent callbacks. Customers hate having to return when their issue isn't resolved. Focus on heading off future issues, not just addressing current ones.
Make use of complaints. Use feedback from disgruntled customers to determine the biggest pain points. Reach out to unsatisfied customers and find out what you can do differently.
Empower the front line. Don't reward speed over quality. Measures such as "average call time" encourage reps to get off the phone quickly rather than fully addressing customer concerns.