Customer service can be sooo difficult at time. Here's my 2 cents worth.

Try not to take things personally. Customers are generally upset because of a situation. Your return policy stinks. You don't have what I want in stock. On and on. Even though the customer might be saying 'you' they really don't mean YOU. They mean the business and you happen to be the one standing there. Yeah it stinks. But remember if it weren't for the customers you wouldn't have the job.

I worked as a cashier at a hardware store for a few years while in college, and yes they can be very rude.