Effective grading begins with clear expectations. Once an instructor has decided what he or she wants students to accomplish in a class, stating those objectives in measurable terms is one of the best ways to make his or her standards accessible to students. When instructors go to the effort of framing their expectations in terms of student learning, and then describing what constitutes a good performance, an acceptable one, and an unacceptable one, students are more likely to understand what they are to do, and at what level they are expected to perform. For instructors, going to the trouble to lay one's expectations out in this way also helps to clarify what the assignments are supposed to teach and minimizes negotiations over grades. The link below on grading essay exams includes a generic guide to what the various letter grades mean in terms of the instructor's expectations. These should be adapted to your personal philosophy about grades.
Grading Indiana University Teaching Handbook
How to Write Better Tests from IU's Bureau of Evaluation Services and Testing
Off Campus Resources
Effective Grading from the University of Oregon Teaching Effectiveness Program