Discuss what constitutes "good writing" for your students. Together (through discussion and/or a write to learn exercise) establish a list of criteria. (Try creating a list on the board. Discuss these, add any of your own, and then create a handout compiling all their input. You can use this during the workshops and when you grade. It gives both you and them an idea of what constitutes each grade.
Ask students to write a letter to you (now seems like a perfect time to do this!) telling you what they particularly want in the way of comments on their papers and how grades affect their writing.
Before you collect the papers, ask students to annotate their final drafts (yes-to write on them! They won't want to...) with a specific prompt. You could ask them to underline their main claims, to predict trouble spots, be a tour guide throughout the paper, trace their writing process, identify the most difficult/easy section to write, etc.
Read all the papers first without writing any comments-then categorize them into what you would (roughly) associate with your letter grade, or other, scale. Reread and write comments.
Write all your comments on a legal pad and then choose which ones you wish to share with the writer on the paper itself. This process works with post-it notes too!
Write your comments on the computer and attach a separate typed sheet to the student's papers (while still making comments in the margins). This approach allows you to keep a copy and also to change your mind or your wording.
Some teachers are addicted to colors both in teaching and their own reading/studying/writing. They might use purple and green ink on student papers: purple for comments on content/ideas and green for rhetorical or style issues. This way students can separate revising comments from simple editing and do both. Plus, it doesn't make the comments look quite so overwhelming.
Try having students respond to your comments after they've reviewed them. Ask students to respond textually in the margins and turn the paper back in. This step will give you sense of how your comments are being taken up as well as insuring that they are read!
Don't try to grade all the papers in one sitting! Give yourself (and your students' work) some space to breathe! Try and return them within a week or two.