Stand Up Straight, Build T'ai Chi Confidence

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Regardless of your confidence level, slouching communicates you lack faith in yourself. 

Try posting a note on the edge of your computer display with a reminder such as an up arrow in thick red marker or the words "SIT UP STRAIGHT". To correct yourself, roll your shoulders back and imagine someone just pulled a string from the top of your head, elongating your spine and raising your chin so it's in a neutral, forward-facing position.

Reference: Simple Tricks to Immediately Boost Your Confidence.

Standing up or sitting up straight, the body perfectly aligned from head to toe, is a T'ai Chi principle.  The joints are not locked, and the posture is not rigid.  It's a relaxed straightness.  In fact, chi is stored in the natural curves of our body, which are seeking straightness during the movement.  Specifically it's about imagining a string attached to the topmost of your head (ba hui), pulling your head, and the rest of your body, up gently.  At the same time, you root yourself, and tuck your buttocks in gently.  So you feel a stretch downward as well.  Your awareness, and mindfulness, and clarity all go up, and that is confidence-building.