We will always have situations where we have to tell people information they don't want to hear: spaces are reserved, we don't have any more copies of that particular book, etc.
One thing that really makes people remember an experience positively even when they aren't getting what they want is offering up alternative solutions. Suggest an alternative location. Explain how to request another copy of that book through interlibrary loan.
Great customer service starts in one place: treating other people the way you would want yourself (and your loved ones) to be treated.
Attitude & Playing the Part
Think about the times you've had great customer service and imitate those behaviors. Play the part of a great customer service provider. Take an interest in the people you're helping!
Know When To Seek Help Yourself
People will ask you questions you can't answer. Ask if they'd like to speak to your supervisor if you can't think of another option. This shows you're taking their need seriously, and opens up another layer of possible options.
Follow Up (Be Accountable)
You may need to hand off a question or concern to your supervisor or the person working the next shift. Make sure you check later to see if the issue was handled, and when appropriate, find out the answer so you can help yourself next time.