Keep in locked safe place. If a test is discovered missing at end of exam, be sure it does not reappear as completed test. Mark grades in grade book prior to returning tests. Warn students that some exams will be photocopied before returning to detect changes. If grades are placed on a computer, insure security is of the highest level.
Place grades on disc, if possible, so that the disc can be safely locked up.
Solution done in a way not covered by instructor. Require oral presentation. Count homework as only a small percentage of final grade or not at all. Give different homework assignments each semester. Ask for oral presentation.