Constructive feedback increases self-awareness, offers options and encourages development, so it is important to learn how to give it effectively.  Constructive feedback does not mean only positive feedback.  Negative feedback, given skilfully, can be just as important and probably more useful.

Destructive feedback is that which is given in an unskilled way that leaves the recipient feeling bad - with seemingly nothing on which to build.

Most people need encouragement and need to be told when they are doing something well.  When offering feedback it can help the recipient to hear first what you liked about their performance.

Our culture tends to emphasise the negative.  The focus is likely to be on mistakes more often than strengths.  In a rush to criticise we may overlook the things we liked.  If the positive is registered first, any negative is more likely to be listened to - and acted upon.

Try to avoid general comments which are not very useful when it comes to developing skills.  Statements such as "You were brilliant!" or "It was awful!" do not give enough detail to be useful sources of learning.  Try to pinpoint what the person did which led you to use the label "brilliant" or "awful".

Specific feedback provides more opportunity for learning.

It is not likely to be helpful to give a person feedback about something over which they have no choice or control.  It is not offering information about which a person can do very much.

If you do offer negative feedback, then do not simply criticise - but suggest what the person could have done differently.  Turn the negative into a positive suggestion.

Tel the person what you saw or heard and the effect it had on you, rather than merely saying that something was "good", "bad", etc.

It is easy to say to the other person, "You are ..."  This suggests that you are stating a universally agreed opinion about that person.  In fact, all we are entitled to give is our own experience of that person at a particular time.  It is important that we take responsibility for the feedback we offer.  Beginning the feedback with "I" or "In my opinion" is a way of avoiding the impression of giving definitive judgements about a person.