Some signs that people may be bored or disinterested in what you're saying:

• Sitting slumped, with heads downcast.
• Gazing at something else, or into space.
• Fidgeting, picking at clothes, or fiddling with pens and phones.
• Writing or doodling.

When you notice that, you can re-engage people by asking a direct question, or by inviting people to contribute an idea.

• Have an open posture. Be relaxed, sit or stand upright and place your hands by your sides. 
• Use a firm handshake. But don't get carried away - you'll likely come across as rude or aggressive.
• Maintain good eye contact. Try to hold the other person's gaze for a few seconds at a time. 
• Avoid touching your face. There's a common perception that people who touch their faces while answering questions are being dishonest.

• Have a positive posture. Sit or stand upright, with your shoulders back and your arms unfolded and at your sides or in front of you.
• Keep your head up. Your head should be upright and level.
• Practice and perfect your posture. Stand in a relaxed manner, with your weight evenly distributed.
• Use open hand gestures. Spread your hands apart, in front of you, with your palms facing slightly toward your audience. This indicates a willingness to communicate and to share ideas.

• Use mirroring. Subtly mirror the body language of the person you're talking to. This will make him feel more at ease. But don't copy every gesture.
• Maintain the appearance of calm by keeping your hands still, and by avoiding fidgeting with your hair or touching your face.
• Look interested. Touching your face or mouth can signal dishonesty. But, it can also demonstrate that you're thinking.