Use Eye Contact
Make sure you make eye contact with the other person. This can portray good faith and honesty. Make sure you don't stare for too long which can be portrayed as aggressive. So when speaking with someone, look directly into their eyes in a polite way and as the conversation continues make sure to stay focused and keep your eyes level.
Have someone take a picture of how you sit or stand. Do you slouch or have hunched shoulders or do you cross your arms? These can make you seem intimidated or defensive. Shoulders should be held back, with your arms hanging loosely by your sides. This simple posture can portray you as relaxed and confident.
I believe one of the best impressions of someone is how they shake your hand. The best handshakes are firm, but not crushing, brief but not terse, and free from nervous sweat. So when meeting someone, make sure you confidently open your hand and make sure you make a positive grip and employ your best handshake technique. If you don't know what a good handshake is go find someone successful and I can almost guarantee they have a great firm handshake.
This is one the skills I have to consciously remind myself to practice. I'm a leg or foot tapper. Unintentionally or not, fidgeting is a sign of nervousness or boredom. So be aware of any fidgeting and give your full attention to the situation you are in.
Control Your Facial Expressions
Your face is usually the focal point in a conversation so what it does can have a huge impact on the outcome of a situation. Your expressions can give away more information than you want. For example, twisting your mouth or pursing your lips may seem like you are holding something back. A slight frown may show that you are unsure, while a smile can show you are pleased.
This can go along with having good posture. You have to move with confident posture as this will go along way to show others a positive perception of your capabilities. Don't forget this can easily be overdone and be seen as arrogant, so be courteous when moving about.