If one kind of error occurs repeatedly, rather than mark each occurrence, mark the first and refer the student to the Writing Center for help with that kind of error. Do the same if there are a very large number of general errors. Once an error is pointed out, the student knows instantly how to correct it. Circle these errors and add a marginal symbol if necessary. If students know you care, they will take more time to avoid careless errors and ornissions.
Index Let the students know what they are doing right.
Index Not every error need be marked. There is a dancer in overmarking. Students may productively attend to half a dozen error notations on a page, but they will be crushed by twenty. We have handouts on these topics that will explain the problems and give more examples.