Elizabeth,
You can certainly "buffer" the situation by pointing out the things they did right first, then what they did wrong, then maybe finish up with one other thing they did right so you close on a positive note. I also use rubrics on the majority of my assignments so that they students have a clear framework that is applied the same to all students and it helps them better understand where they lost points and what needs to be improved. However, even then, students sometimes still need a little more clarification.
Herbert Brown III
