How curious, yesterday, I was in the bookstore reading a psychology magazine and they had an article about Charisma.
Their key point has already been mentioned here, but I'll put it in their context. The most important part about Charisma is to be a good listener. You need to be able to tell what emotional state the other person is in or is going through. Then you need to be able to tailor your reponses to help meet their needs or to find the commonality between you so that they know that you "get" them. Most people talk to fulfill their own needs, being charismatic is about paying attention to them rather than focusing solely on your own needs.
Open postures are postures that make a person more vulnerable. Usually they are more spread out and take up more space. Closed postures shrink the person and create barriers between the world and the person. When people cross their legs/arms, stand with their feet close together (as opposed to shoulder width), or with their hands in their pockets, they are communicating that they are insecure at that moment. They gain a feeling of security because there are extra barriers between them and the word to protect them. The fetal position is the epitome of this. Confident people don't need those extra layers of protection because they know they can handle the issues that might arise in their given situation.
When standing, keep your legs far enough apart to create a stable base. Stay balanced and don't lean more on one foot than the other. Stand straight up and stay relaxed. Truefire explained it well. When sitting in an unprofessional setting, take up space. Spread you legs apart rather than crossing them and control more of the space than you normally would.