1. Colours
To assert your authority, wear red. Red makes you look and feel confident and in control and gives you an energy boost. Avoid it, however, if you want others to open up to you. you'll intimidate them. Wear green to help you concentrate - it will focus your attention on what needs to be done. Blue will calm you in times of stress, but avoid it if you want to be creative. Purple is a brain-booster, so wear that when you need to be bursting with ideas. Black gets you taken seriously, but can make you conform to the corporate stereotype - it'd be hard to stand out or have any flair.
2. Look Sharp
You'll win points if you look fascinated by what your boss is saying. Lean forward a little, cock your head slightly to one side and make eye contact for a few seconds. Don't let your eyes wander above their head or they'll know you're faking it. If you're in a meeting, or a panel interview, don't try to make eye contact with everyone at once - it will affect your ability to concentrate on what you're saying. The rule of thumb is to focus on the last person that spoke.
3. Hands
When you're talking to your boss, keep your hands where they can see them. Nobody's afraid you're going to brandish a blade and try to stab your supervisor, but you're boss' subconscious mind could register your hidden hands as a sign you're untrustworthy. That doesn't mean you should wave them about as you talk either - Wild gesticulation will merely attract attention to your hands rather than the content of your speech.