Prepare your attitude: You CAN help or solve the issue. It's not magic -- there's always an explanation. Don't try to fix it right away, just try to narrow it down. Don't panic. Don't get mad. Be patient and don't skip steps.
Let the customer speak till they are done, repeat back to the customer exactly what you heard and some clarification if necessary. This ensures you know what the customers goals are and the customer knows you know what their goals for the call are.
Avoiding interruptions is number 1 when helping a customer.