Common courtesy is a small gesture that makes a big difference and should be practiced as often as possible.
An essential component of common courtesy is awareness and common sense--looking outside yourself to see when someone needs help or acknowledgment. As a courteous person, you are aware that you are walking into a room full of people or that your waiter has arrived to take your order. Then, awareness leads to action. It is usually quite clear what needs to be done--open the door for the woman holding the baby, move your car up two feet so another person can park behind you, acknowledge your sister's shy boyfriend with a smile and some conversation, apologize if you bump into someone. A third component is to give courtesy freely, without expecting anything in return. People may not even take notice, much less return the kindness, but you can take heart in the fact that you are creating the kind of world you want to live in with your actions.
When you are out in the world, remember to be aware of others, lend your hand when one is needed, and give this help without an ulterior motive. Through these small actions, you make this world a better place in which to live.