Some people are seen as aggressive simply because of the way they naturally stand. A man who is standing straight, feet slightly spread, and with their arms crossed or at their hips will appear to be threatening even if only on a subconscious level.
Be aware of your posture and what it is saying to those around you. If a person feels threatened they will keep their guard up which is bad for negotiations. Keep your hands in a neutral position such as in your pockets, slouch a bit or lean against an object, and tilt your head slightly when listening. Think of yourself as a friendly professor rather than as a prize fighter and watch how your posture changes and the effect it has on those around you.