Go out of your way to make positive oral and written comments. Please read the student papers prior to their presentation. When you can see ways to improve their work, share that as positively as possible. If the student has made many mistakes, stress the improvement of the main faults. Most of us can only handle one problem at a time.
Some minor problems exist, but much effort and care has been taken by the student.
A paper or essay that you can't really fault. The work is really complete in every sense of the word. For example, a student studies the motion of a puck over an air table. The student never indicates any consideration or mention of friction. That probably would be viewed as a major mistake.