First of all, ask yourself: "What do you want to accomplish by telling the truth?".

for yourself you have to understand that telling the truth, you do not want to offend people, but want the positive changes.You need that person would have heard and changed their behavior.

Based on this position, and build your conversation.
Since you want positive change, his conversation also build in a positive way.
In his conversation, avoid displays of negative emotions.Do not yell at him, especially not insult him.
not go to the individual.You have to show the person that you do not like his actions, and not himself, as a person.Thus, we should not focus on the identity of the interlocutor, and his behavior or a specific event.

not disturb self-esteem interlocutor.
Build your speech so that the other person understand that you "do not find fault with him," and konstatiruete existing facts.At the same time you have to be absolutely objective.
expressed its willingness to open a conversation.Give the other person understand that he can openly explain to you the reasons for the incident.
openly express their feelings about what is happening, but, in any case, do not give praise.For example: "I am very disappointed that you did not do it," but "you - a liar!".This assessment will lead to a breach of contact and counter-accusations.
tell the other party what you want from him in the future.Your speech should contain the desire but not the order.For example: "Please, next time do not do this."
If you follow these simple rules, chances are your frank conversation will change the situation in a positive direction, and you keep a good relationship with someone.