When I am grading a student's work I am looking for certain criteria. The student's are well aware of what this criterion is because it is posted on the board at the beginning of each project. To me each assignment has it's own set of requirements so I break their grade into points based on 100%. Below is a sample of a grading rubric for the picture found on this page.
GRADE FOR THE MONSTER WITHIN
Participation (did you stop me from helping other students) 10 pts __________
Effort (you tried your best) 10 pts __________
Finished on time 5pts __________
Construction of paper mache (it stands on its own, paper isn't
sticking out, it has more than one layer) 25pts __________
Painting Techniques (is it neat with clean colors) 25pts __________
Added Details (trims, fabric, hair, etc.) 15pts __________
Included personal written evaluation 10pts __________
TOTAL 100 POINTS
I always try to write a positive comment in this section about some aspect of their work.