How do I reduce careless errors on the GRE?

Careless errors don't go away by "just being more careful." You need specific structural habits: cross your 7s so they don't look like 2s, write the question number before you start working, re-read the entire text completion sentence with your answer plugged in before moving on, and always re-confirm that the other answer choices are wrong. These are deliberate, repeatable processes, not vague intentions.