Being considerate involves awareness of other people's perspective, and action to make life better for others, not just for oneself. Here are some examples of things one can do, and what not to do, all taken from everyday life.
1. Be tidy. Imagine that public places are your own private living room, just minutes before you're going to have visitors you want to impress. You wouldn't want litter all over the floor, chewing gum stuck where people can tread or sit on it, or graffiti with names or slogans painted on the walls.
2. Be efficient. When you are queueing (waiting in line), you can save time for people after you if you have your money (or bus pass, or tickets) ready when required, instead of just starting to search your bag or pockets when asked.
3. Be aware of space use. When you stop to look in shop windows or talk to people, make sure you are not blocking the whole of the pavement (sidewalk) or walk space. This is particularly relevant if you have a child in a pram (stroller). More importantly, don't block the area at the top or bottom of escalators. When people reach the end of an escalator, they have to step off it, and if the space in front is blocked, this could cause a pile-up of people who have no other option but to move forward.
4. Be quiet. Public spaces are full of noise - traffic, conversation, in some places music. You can reduce the total noise by having a relatively quiet ring tone on your phone, and limiting your conversations when in public. People riding with you on the train or bus don't want to spend the entire journey hearing you talking. If you are with someone else, make them the priority, and try to postpone any phone conversations you get. It's more polite to tell someone "I'll phone you back" than to give the phone call precedence over the person you are with.
5. Be health-conscious. Some of the things people do in public can harm the health of others. Specifically, smoking and leaving the car engine running. Personally, I am strongly opposed to smoking and would be overjoyed if everyone in the world could give up. However, people who do smoke can still become considerate of others by being aware of how the smoke spreads. Don't smoke where it is forbidden by law, and try to find somewhere out of doors, far from other people. I can smell smoke from 10-20 meters away, and sometimes have to move quite a long way from where I was standing just to avoid it. Also, be aware that a strong smell of smoke sticks to the smoker for several minutes after smoking. People who throw away their cigarettes just before getting on a bus or train are still exhaling smoke inside the carriage... As for leaving the car engine running while the car is stopped (some people do this to keep the air conditioning on while they wait for someone), this causes pollution which is bad for everyone's health and also for the environment. Switch off the engine when you park the car.