Lifting something heavy without good technique is one of the most common ways to injure your back. Many times people lift with their back instead of with their legs. This video will show you two different ways to properly lift a moderately heavy object.
As seen in the knee strengthening video, you want to make sure you knees do not go over your toes. Spread your feet about shoulder width apart, and again, make sure your knees do not go in front of your toes. Stick your butt back like you are going to sit in a chair, but try to keep your back straight while bringing your chest forward. Pick up the box or object in a squatted position, and lift with your legs!
The lunge technique is similar to the squat in that you want to make sure your front knee does not go in front of your toes. Keep your back straight, and lean your chest slightly forward so you can pick up the box or object. Lift with your legs!