First, assume a non-threatening but still defensive posture. Take a small step back with your dominant foot (it's the same side as your dominant hand) and hold your hands open in front of you, with the palms facing your antagonist, at about chest height. Speak calmly with the person, and do not shout back (though you may have to raise the volume of your voice to be heard). This will allow you to be on guard without being provocative, and give you a bit more distance.
Keep your eyes open for signs of an imminent attack. These may include the person's face going from red and flushed to pale white or a sudden dilation of the pupils (signs that the sympathetic nervous system has engaged), clenching the fists or turning to the side (both signs of an intent to strike), suddenly looking around (for witnesses), or suddenly becoming quiet. If any of these things happen, or any other abrupt change in behavior occurs, be ready to fight.