Rubrics provide clear guidelines both for you and your students.
What is a rubric? Essentially, it's a list of your objectives for an assignment or exam essay, with some way to measure the quality of students' responses to each objective. Your rubrics should reflect the things you think are most important. They can be as simple as a list of points you'd like to see in a student's essay or as complex as a complete outline of what you expect students to produce. But it's always your choice.
First, decide what each assignment is meant to accomplish. What are the students supposed to learn by doing this paper? Is it about learning how to do background research, how to design a research project, how to communicate about your subject to a professional audience or the public, or something else entirely? If you can't articulate the purpose of the assignment, perhaps you're not ready to assign anything.
Once you've decided on what the assignment is meant to teach, create a specific rubric for what you expect from your students. This might be fairly general for a simple assignment, or quite detailed for a final report on students' research. The more specific you can be, the easier it will be to explain the assignment to your students. And even better, the easier it will be for you to grade it! You'll find some sample rubrics that I use for my own courses among the links at the left side of this page.
Give the rubric to your students
By giving the rubric to your students before they begin their assignments, you make it possible for them to understand exactly what you want done, and how. Be sure to let them know that you'll use these criteria to grade them, and that they can use your rubric as a guide to writing a paper that will earn an A.
I always post my rubrics on my website, linked to the assignments for which they're designed. My students can download the rubrics and refer to them as they work on each assignment, checking to be sure they've satisfied the specific requirements for each part of the paper or project. In addition, many of my former students have given me permission to post their papers as examples. Current students can look at these successful (they all got A grades) efforts and model their own work on those.
When grading, I use the rubrics as checklists, assigning points directly on them. This speeds up grading tremendously. As I read each section of a paper, I know specifically what I'm looking for, and what constitutes appropriate quality. I'll still make comments in the papers, but those are more for guiding the writer's thinking, and not for grading. I return each paper with a copy of the graded rubric, so students can see precisely where they did well and why any points were taken off. This saves a LOT of questions and unhappiness on everyone's parts.
Tools and Samples
You'll find several of my own rubrics among the links at the left side of this page. Please feel free to use them or modify them for your own courses. You'll also find a link to some information about Bloom's Taxonomy of Educational Objectives, which you may find helpful in developing your criteria. There are several good websites that provide information about rubric design, and you'll see links to those, as well. As always, I'd be happy to help if you have any questions.