This one cries out for the "natural consequences" approach. Next time you have someone who is not learning proper lifting techniques after repeated instructions then make them a casualty of the scenario. Declare them injured with disabling back pain and make the rest of the team figure out how to handle the extra patient. That should make the point about the potential danger to the one who is not learning and to the team as well as apply a healthy dose of peer pressure on the individual who isn't getting the lesson. Now the team has one less member to help in the extraction and one more patient to be extracted.