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. Compare solutions with manual. Careful grading-look for similarities. Count homework as only a small percentage of final grade or not at all.