You can learn a lot from the body language of others but its usefulness doesn't end there. As social psychologist Amy Cuddy points out, the body language you choose to use can influence your own behaviour.
People with powerful body language -- movements that tend to be more open and spread out, that take up more space -- also feel confidence. They're more likely to take risks, feel optimistic, and even produce less cortisol (the stress hormone) and more testosterone (the dominance hormone).
Although we've known for a while that displaying dominance through body language contributed to a more powerful appearance, Cuddy found that lower levels of stress were also a major contributory factor. She, and her partners, brought people into a lab and asked them to adopt both high- and low-power poses to find out if simply faking confidence (and displaying a lack thereof) had psychological and physiological effects. The study found that faking high-power poses caused people to become more confident and willing to take risks, but that their testosterone levels rose and their cortisol levels decreased significantly. Conversely, low-power poses cause the exact opposite reaction.
The results of the study suggest that you want to sprawl out in stressful situations to feel more powerful, but kicking your feet up during, say, a job interview definitely sends the wrong signal. Cuddy explains that it isn't so much the body language during high-stress moments that matter, but rather the nonverbal signals you create prior. In the event of a job interview, she found that opening up your body for a few minutes prior -- even if it's privately in the bathroom -- can make a big difference.
Some people may take issue with the idea of "fake it 'til you make it" because they don't want to always feel like a fraud, but Cuddy explains that it's more of a process of faking it until you become it. Feeling powerful and confident is a process and something you can learn. It starts with small changes in your body language -- something everyone can do.
Be sure to watch Amy's TED talk above for more.