I just spent a little bit of time watching pitches and interviews that I've done over the past couple years just to take a look at my body language and the way I carried myself. As I've seen myself change, I think there's a couple things that really stuck out in terms of body language that helped me carry myself with a little more confidence.
Don't cross your arms. Just don't do it when you're doing an interview or you're on a stage.
Relax your shoulders so you feel comfortable. This will help you stay more in the moment and give strong answers.
When standing, lean forward to emphasize a point.
Take up physical space to show confidence. This means don't be afraid to get your elbows off your sides.
Monitor your microexpressions - that's the little things you do immediately after you finish a sentence. Practice giving answers in front of a camera so you can get your microexpressions to convey confidence and calm. Here's a decent article to get you started: http://www.scienceofpeople.com/2013/09/guide-reading-microexpressions/