Strange as it may sound, your body language has a huge impact on your level of confidence. When you sit or stand with your shoulders hunched, head tiled down and body compacted, you subconsciously begin to think and act in more submission, passive manners. If you take a more dominant posture, however, you feel more confident and in control.
There are a variety of power postures, but the best known require you to straighten your spine and lift your gaze from the ground. Then, stand with your feet shoulder width apart and either throw your arms up in a v-shape like you are celebrating a great victory or puff out your chest and stand with your hands on your hips like a superhero.
It does not take a long time for power postures to begin to affect you. Stand for a minute or two in the morning in a power posture or take up the posture in the bathroom before an interview. These are small instances, but they will add up overtime.