We tend to think of body language as something that expresses our internal states to the outside world. But it also works the other way around: the position of our body also influences our mind.
If you want to feel more powerful then adopt a powerful posture. Carney et al. (2010) found that when people stood or sat in powerful poses for one minute—those involving open limbs and expansive gestures—they not only felt more powerful but had increased levels of testosterone flooding their systems. Powerful poses take up more space, so spread your body and open up the arms or legs. When you dominate the space, your mind gets the message.