Correct Without Being Critical
Do not be directly critical of others because this will usually cause them to be defensive. People who do things that are wrong often rationalize that what they have done isn't really wrong. Even the toughest criminals, such as the worst murderers, don't like to admit having done something wrong!
There is a major difference between being critical, and having a positive influence on others by saying things with compassion and true caring. When you sound critical, the person on the receiving end is likely to deny your words, which will be perceived as an attack. And then you won't accomplish anything.