How exactly should you lift something heavy without hurting your back?
The principle of lifting is the same whether the object is heavy or light. Many people have "put out" their backs just by picking up a piece of paper off the floor, for example.
When bending to pick something up, you should always use your legs to bear the weight, not your back. This means you should not bend at the waist when lifting.
Stand straight and bend at the knees as you lower yourself closer to the ground. Pick up the object without bending at the waist. If the object is heavy, be sure to bring it as close to your body as is possible so your body absorbs the weight and not the your arms and upper body.
Once you have the object securely and safely in your arms, move back up to a standing position, always using your leg muscles to do the work.
Jens Ivar Brox, MD, PhD, et al. Evidence-informed management of chronic low back pain with back schools, brief education, and fear avoidance training. In The Spine Journal. January/February 2008. Vol. 8. Issue 1. Pp. 28-39.