When you tell someone that they are useless at something, have no gift for it, or are doing it all wrong, you successfully destroy all their incentive to improve. No matter how true they are, refrain from using such comments, as they are extremely demoralizing and are a huge blow to a person’s ego.
So, do the opposite. Encourage them to excel, and you will motivate them to work hard, as they will now want to be worthy of your praise.
When others fail, encourage them to try again.
Let them know that you have faith in their ability, that they have an undeveloped flair for it, etc.