How to prevent neck and shoulder pain from heavy lifting work?

Body Mechanics: Your spine sees a lot of forces with lifting and twisting. You must use proper mechanics when lifting and always be aware of your position in space. It helps to have a strong core, the muscles that support your abdomen, chest and spine. I would consult with a physical therapist familiar with work injuries. You need your work space to be evaluated and your body evaluated for areas of weakness

Prevent or treat?: To prevent it you need to limit the weight lifting or limit the repetitions. To treat it i suggest heat and massage. You could consider adding a muscle relaxer.