1. Ask how you can fix the issue. People love to talk about their experiences. So, why not let them? Instead of just rushing to fix a problem how you see fit, take a second to listen to the customer's complaint. Then ask him or her how you can fix it. If the request is reasonable- do it. If the request is unreasonable then suggest what you can do.
2. Respond. Ok, this seems like a very simple and obvious step. I have run into issue after issue where corporate executives fail to respond to me. I know I make phone calls. I know I send emails. I often hear nothing back from the addressee. YOU need to handle any and all issues that come before you- this includes following up with the customer once the issue has been resolved.
These two easy steps for customer service act as the key to client happiness. If you can maintain this strategy, you will always have happy customers.