When speaking with customers, especially challenging customers, it is easy to forget that you are there to help them work through any issues or requests that they may have. Being mindful of the words and statements you are making will ensure that you will not offend your customer or sound accusatory. A good way to avoid coming off offensive or unhelpful is by using "I" statements, instead of "you" statements. By changing the way you phrase a statement you can avoid sounding like you are accusing your patron or challenging their patience and intelligence. Some examples of how you can turn a negative statement into a postive one are:

Negative: "You're confusing me."
Why is this wrong? Because this sounds like you are accusing the customer for your confusion, but you're not allowing them to explain the situation.
Positive: "I'm confused. Can you further explain the situation to me so that I can understand?"

Negative: "You have to.../you should..."
Why is this wrong? Because the customer called you to help them. If they haven't already found the information they are looking for, then most likely they will not be able to find it without your guidance.
Positive: "Let me help you..."

Negative: "You haven't heard about our product?"
Why is this wrong?  Because the customer contacted you in order to become more familiar with the products, so it is best to take the time to help them with any questions that they may have.
Positive: "Since you are unfamiliar with our products, let me take some time to explain them to you and answer any questions you may have."

Negative: "You don't seem to understand."
Why is this wrong? Because this has the potential to offend a customer, and the real problem may be that you as the customer service agent are not doing a sufficient job of explaining.
Positive: "What can I clarify?"

Customer service can be challenging, but being mindful of your words can not only result in happy customers, but return customers!