I once worked for a man who did just two things to me. Not for me. Not with me... but to me.  He lied to me and he lied about me. 

This idea comes from a Zig Ziglar quote... If you always tell the truth, you will have nothing to hide. Telling the truth tactfully is an artform. It should never include half-truths or avoidance. 

Best rule of thumb is simple: be a person of integrity at all times. You can do it in a way that builds respect with everyone you deal with.