How can I keep from hurting my lower back?
Every time you bend over to pick up something, even a pencil, you put your back at risk. To reduce the risk of injury, you need to learn good body mechanics. That means moving so that your back is moving as it should, without putting stress on any particular section.
First, when lifting anything, be sure to bend your knees and keep you back straight. Do not bend over, keeping your legs straight. By bending your knees, you're lowering your body's center of gravity, and you are letting your leg muscles bear the weight of your body and the item you are picking up.
If you're lifting something heavy, hold it as close to your body as you can, don't stretch out. Finally, always keep your body straight, don't turn at the waist if you're carrying or lifting an object. If you need to move a box from the floor in front of you to the counter beside you, bend at the knees, keep your back straight, pick up the box and then stand up straight again. Then, turn with your feet, not at your waist. Move your body so it is facing where you'll put the box down.
If you often lift heavy objects, consider getting a special belt that helps support your lower back when you lift.
Helma I Jzelenberg, MSc, et al. Effectiveness of a Back Pain Prevention Program. In Spine. Vol.32. No. 7. Pp. 711-719.