I have learned that telling the truth at all cost creates a kind of force field around you that protects you from all evil. However, there is  a down side.

Since you tell the truth, you tend to think that others also tell the truth. That is often a mistake. But there is a principle that can help in this kind of situation. Once you have heard someone tell a lie, then you already know they will tell more lies. This applies to slander also. Once you have heard someone say slander, then you can be sure they will slander you too when you are not around .