Treat every client/customer/patron as if they were the CEO of your company. Don't discriminate based on age, gender, dress, appearance or speech. You can't really tell who you are dealing with based on externals.
If you are alone, busy with one customer or a task you can't stop in the middle of, and a customer approaches the desk, acknowledge that new customer's presence with a glance their way and a quick, "I'll be with you in a moment/I'll be with you as soon I have finished with this customer." People are usually much better about waiting if they feel their presence has been recognized and they know they aren't being ignored.
When the customer has a problem, always offer some sort of way to rectify it. It might not be exactly what the customer wants, but give them *something*, even if only a form to fill out a complaint. It is incredibly frustrating to deal with a CSR who keeps saying, "Nope, can't do that," without offering any possible way to fix things. IMO, it's this feeling of being stonewalled that causes people to get upset, moreso than the actual problem itself. A sense that the CSR is trying to help, even if they can't, goes a long way.
If you are allowed personal calls when things are slow, end them when a customer approaches. Don't turn your back and leave the customer waiting while you chat. Same with conversations with co-workers. You should put the conversation on hold while you deal with the customer, not ring the customer up with your head over your shoulder talking to someone else.
Bear in mind that any customer could be new to your store or service. You may be very familiar with the store, the company, the policies, but the person standing in front of you/on the phone may have no clue. Don't assume that the customer knows the jargon of your trade/company. Ask them if they understand what the next step is/where to go, etc.