Purchase a small pad like a stenographers notebook that you can always carry around.
Spend some time at a place where you can observe a group of people, or where there's a steady stream of people like a bus stop, restaurant, park or sporting event.
Write down detailed descriptions of people. When you see someone who you find intriguing, whether it's something about his mannerisms, the way he's dressed, his facial expression, or anything else, put it in your notebook. Details might include the way the person walks or sits and his approximate age.
Take notes on your surroundings to generate ideas of potential story setting. Note the time of day, the season, and the types of people inhabiting the area, using descriptive phrases or short blurbs that incorporate the smells, noises, and sounds around you. Don't worry about writing down names of streets or other specific places unless you find the name catchy (or you're writing a factual article or nonfiction book).
Observe and jot down interactions between people. You might come up with an idea for a story or novel plot from watching how people interact within a particular setting or environment, or how they affect the environment.
Consider your own life. Generate plot ideas or a characters backgrounds or jobs based on your own career and the people you see everyday. You can use your knowledge and work experience to write a fiction story or a nonfiction piece for a trade journal, for example.
Get ideas from places you go. Jot down ideas for locations, characters and plots when you travel for business or vacation, or even just visit a local museum. Also make note of factual information and observations from these places, as you can use them to write a nonfiction article.
Attend area events. County or state fairs, craft fairs, and amusement parks provide interesting fodder for articles.
Research specific people and events. Take advantage of library resources and museum archives to make notes and generate ideas for characters and plot or settings.