Bedford St. Martins has a post from Andrea Lunsford, director of writing and rhetoric at Stanford, on responding to student writing.
First, Iâ€™d make a distinction between responding and gradingâ€”and put most of my emphasis (and time) on responding. Grading is just the final act of assessing the state of the draft, comparing it to the criteria you have established for the assignment, and then giving the appropriate grade or the number of points: this part of the process shouldnâ€™t take long at all. Responding, on the other hand, means engaging with the studentâ€™s ideas as well as the structure, syntax, and style of an essay. This is where all the time goes.
Then she gives some specific suggestions.
She has other tips that are linked off that page. Just look on the right side.