Don't Always Apologize

A very useful psychological trick is to never apologize if it's possible to thank or compliment. Let's say a customer has become visibly impatient from waiting. Whilst bringing the customer their order, try thanking them for being such an nice and understanding person.

In order to keep their image of a "nice and understanding person" they almost certainly won't complain even if they had intended to do so.

The act of apologizing is essentially admitting to a mistake. Although it definitely takes strength to show such a weakness, it's often just not the correct option.