Never take your bad day out on a customer. One of the most important keys to your success is a friendly personality. You don't have to be an outgoing person, but you must be friendly. I supervised two persons in my last job. I would never hire a person who did not display a friendly disposition in the first interview.
Unfriendly people are always boring and never fun to work with.
They are also chronic complainers. If you deal with a bad customer, put your smile back on for the next one. If you remember nothing else about these tips, remember this one. 5. Do not rely on the Spell Checker. It is not perfect either.