Speak the truth to yourself and others. Take responsibility for your own actions, for who you are. If you promise something to others, deliver, and on time. If you promise something to yourself, deliver, without procrastination or excuses. If you stumble or fall, do not wallow in the mud of being a victim, instead, pick yourself up.
Authenticity has special applications when dealing with people, especially difficult people.
One of my coaching clients was having trouble being recognized by her boss. The boss would actually pass her in the hall and not even nod hello. My client would boil with rage at this. The solution was clear. I explained that if she came into the situation exuding her own stature and authenticity, the message would get through without words.