This is not just about your attitude, but also your physical manifestation.
Source: Sales author Grant Cardone
For the next week, practice smiling with everyone in every situation you encounter. Do this until you are able to argue with a smile, disagree with a smile, negotiate, overcome objections and close with a smile.
Have you ever noticed that very successful people are smiling all the time? It is not because they are successful that they are smiling, it's how they became successful.