Stretch slowly, and stop if you experience sharp pain.
When lifting, maneuver the object close to your body, and use the strength in your legs to get the object off the ground, rather than your low back.
Have you ever heard of anyone straining a thigh muscle while lifting? Probably not. That is because the muscles in the legs are longer, stronger and more resistant to strain. The muscles and ligaments in the back are shorter and prone to muscle spasm.
Start with one knee on the floor, use the strength of your arms to raise the object up onto your mid-thigh, then use the power of your legs to stand up. An alternate method is to bend both knees in a squatting position, grasp the object keeping fingers underneath it, keep your back erect and stand up. In both examples, use your leg muscles, not your back, to generate the lifting force.