Set expectations upfront
It is worth producing a Service Level Agreement and publishing it. This way customers will know what to expect from customer services. Remember though to stick to your SLA, if you say you will return all calls within 2 hours then this is something you must do.
Also, don't hide your support contact information in an obscure page of the website - this will only put the customer in a bad mood before they even talk to support. It's important to be upfront about any charges for the service as well.
Put yourself in their shoes
Employees working within support must show empathy with the customer, have strong interpersonal skills, and above all, patience. Make sure they receive adequate and regular training on the products and services you provide. Imagine how irritating it is for a customer if they find they know more than the call centre agent.
Sometimes your customers may overstep the boundaries of what you support. You need people who can be strong, but yet polite when saying no.