Bending your knees is perhaps the single most important rule when lifting moderate to heavy objects. Take a tip from professional weight lifters. They can lift tremendous amounts of weight because they lift with their legs, not their backs. When lifting a crate or a box, your feet should be placed close to the object. Center yourself over the load, then bend your knees and get a good hand hold. Lift straight up, smoothly. Allow your legs to do the work, not your back.