One of the hardest things when communicating with customers is not taking it personally. Don't take criticisms or complaints as personal attacks and above all else resist the temptation to try and "one-up" your customer.

Loads of times we've received pretty insulting emails, but done our best to be polite and helpful back, more times then not we've received a really thankful and praising email back in response. A lot of times customers forgot they're sending a message to a real person, and just think they're contacting a faceless corporation.