You say more than you ever knew in the blink of an eye. A glance, a turn of the head, a touching gesture - one arch of the brow says more than a thousand words.
Samy Molcho (Body Speech)
Children acquire socially appropriate body language along with speech. While much of the secret language that they once used goes underground, they learn new gestures and motions that are appropriate for the culture in which they grow up. Few children are taught to lift a brow at the end of a question to indicate curiosity - but most learn to do it naturally. In Japan, children learn that it is polite to lower their eyes when greeting someone of higher status. No one tells them to do it - they learn it by observing those around them. In most Western countries, boys (and increasingly these days) learn to shake hands in greeting.
For those of us who grew up in the West, shaking hands is a natural thing. We don't have to think about whether to put out a hand when greeting someone, or who should take whose hand first. It's all learned behavior - but it's coded into us by decades of observation and experience. But when Japanese employment companies started trying to place native Japanese in Western companies - gaishikei - they found that the manners of Japan put their candidates at a distinct disadvantage. The lowered eyes that denote respect in Japanese society are interpreted as lack of confidence and shiftiness in Western companies. The employment agencies found that they had to teach their candidates how to alter their body language to fit into the gaishikei before they could be accepted by their new employers.
Learning body language isn't quite as easy as it sounds. There's far more to reading someone accurately than just knowing that a touch to the lips may indicate lying or that widening of the eyes shows interest. Most sociologists that study kinesics caution that you should never judge someone just on one gesture or movement, that every interaction is made up of a combination of words, gestures, facial expression and body positioning. To focus on one part while discounting the others is a surefire way to misinterpret what you're seeing.
Becoming proficient at reading and using body language requires both self-criticism and tolerance of others. You'll need to study your own movements and postures with a critical eye, and be willing to look for alternate meanings to what you think others are saying. You'll need to be aware of how your gestures and attitude may be completely contradicting the message you're trying to get across.
In addition, you'll need to learn to 'listen with your eyes'. Listening well is more than half the battle in effective communication and it doesn't matter whether that communication is a speech before the board of directors or a pickup line at the local bar. If you're not actively listening to your audience, you can't effectively deliver your message. To put it another way, if you're not paying attention to the signs, how will you know when to kiss the girl? If you ARE paying attention, you won't have to wonder - she'll tell you, but it won't be with words.