Honesty goes far deeper than simply telling the truth. Honesty possibly has as much to do with omitting sentences you would otherwise say as it does with adding them.
I am currently experimenting in honesty and the benefits in various categories, including benefits to myself, to the people I speak to directly, and to the observers or witnesses and then by extention to a greater community. For me an important role in this is played by something I learned last night is called "cognitive thinking".  This idea extends far beyond speaking what you believe to be true, but assessing the benefits of saying that, and of analyzing your own thoughts as they are being processed. It is important to be fully conscious and aware of what you are thinking whenever possible, whenever you think to do it.
By picking apart the influences in my life which have led me to certain conclusions, it is easier for me to critique how valid these conclusions are. There is no need to feel ashamed of your thoughts, no need to be embarassed by judgements you pass on people based on initial reactions, but if you can acknowledge those thoughts and move past them, it is unnecessary to be locked into those ways of thinking.
If I am acting in a way that is contrary to my values and beliefs, I consider that not being honest to myself.
Right now I am discovering how much is appropriate to say, and when.