If you put the fear of g-d into them during the classroom session, and advise them that they will be forking over $$$€€€ if they strike someone with the lift trucks, then looking over their shoulders, when reversing, won't be a problem.

Biggest issue with my students is failing to neutralize the forklift prior to utilizing the control lever, for example, leaving the truck in gear prior to utilizing the control levers. The bad habit experienced operators are the worse but the newbies follow instructions and get it right.