Here are sample marginal comments that help students get a clearer sense of how you are reading their work:
Your evidence in this paragraph is effective -- it convinces me to believe your claims.
Where do you explain how you came to this conclusion?
How could you use this information to target your readers more effectively?
Read this sentence out loud. Can you think of a way to rephrase it?
Providing these types of comments help you to clue writers into what their audience is thinking as they read the paper. Its also a good idea to make use of these comments in the summary/end note you provide students or in the conferences you hold with them. This kind of follow up provides these marginal comments a context and makes them more useful for students as they learn to be stronger writers.