Perhaps the single most important element of designing a rubric is to ask yourself what you want to emphasize as you read and respond to the writing assignment. This question is key to developing a good writing assignment in the first place, and it's also key to constructing a useful rubric. If at all possible, create the rubric as part of your work developing the writing assignment per se. Students appreciate seeing the rubric early on as a way to focus on key components of the assignment, and you'll be sure that you emphasize the same points as you present the assignment to your students and as you evaluate papers.
In brief, here are workable steps to develop an effective scoring rubric.