- give criticism in private
- it usually should be possible for the person receiving the criticism to improve (constructive criticism): e.g. "your problem is you are no good with people" is of some value to the recipient in that it highlights what you believe are their strengths and weaknesses, but it's not very constructive in that it doesn't suggest any way for them to improve. However, imo even this sort of non-constructive criticism may be valuable if the recipient is asking something like "why didn't you choose me for this position?".
- make the criticism about a behavior, not about the person: a bad example is the previous one, "your problem is you are no good with people" which suggests that the problem is an inherent, unchangable property of the person being addressed.
- complaint sandwich: give compliments and/or affirmation's of the person's value before and after the criticism
- only give criticism when asked (or as part of a formal procedure for criticizing, e.g. a teacher grading an essay, an annual review at work)