Students themselves are a wealth of information when writing report cards. Ask students to complete this Student Self-Evaluation. It can be very telling to see where you and the student agree or even disagree on how they are working/participating in class. This is a great conference conversation starter, as well, and can provide specific information for the comments section of the report card.
Use the Student Self-Evaluation in conjunction with our Observation Checklist found here to help you write meaningful report card comments.