I know it's important to bend my knees when I lift heavy things. But I've had surgery on both of my knees, and they really hurt when they bend too far. How can I protect my knees and my back when I lift?

That can be difficult. It would be best to lift heavy items from waist level so you don't have to bend your knees. If you have to lift heavy items from the ground, try to bend your knees only as far as they can go without hurting. Then lower your body by bending at the hips and keeping your hips back. Your low back should be arched in slightly. Grab the item without rounding your back. Without holding your breath, tense your abdominal muscles and lift. Straighten up from your hips. The idea is that your hip muscles will take most of the load, rather than your knees or low back.