If You're Determined To Mark Errors, Try This Approach
Write out your hierarchy of errors. You can base your hierarchy on pet peeves or you can use one that ranks errors by how disruptive they are for readers.
On the first paper you assign, mark only the error at the top of the list. Circle the error where it first appears. Provide an explanation in the margin. (Some teachers write explanations and store them as separate word processing files. They then print out only those explanations pertinent to a given paper.) When the error appears again in the paper, circle it again but don't provide any additional explanation.
On the second paper you assign, put an X in the margin where the error that you explained on paper 1 appears in the second paper. Then move to the next error or two in your ordered list. Circle and explain those errors.
Depending on how many papers you assign, you can work well down your hierarchy of error.
Why adopt this approach rather than the "mark everything" approach? Two reasons:
Marking everything takes too much time and is more likely to confuse than help the student. You'll get more and more upset as the errors continue. Students are more likely than not to dismiss your marks because you've now fixed the paper so they don't have to.
When you indicate that an error occurs but you don't show the student where exactly they've stumbled, the student will look more closely at the sentence. Only when students take the time to analyze the problem are they likely to learn to correct it. Eventually, especially if you require that students edit for themselves (either as a follow-up exercise or on the next paper), students will learn to look for the errors identified this way before they turn papers in.
For students with multiple errors, one-on-one instruction tends to work much better than large-group instruction.