keep the back as straight as possible
People always say "keep your back straight". Actually that's impossible as the human spine is in a double S shape configuration. It's better to try and keep the back as straight as possible as indicated by this slide.
Imagine a line drawn through the hip, the top of the arm and the ear as shown here. If this is approximately a straight line it will take excess pressure off the bones in the spine which do the most work in the lumbar region.
In order to achieve this, try and do the following:
Bend your hips, knees and ankles so that one foot is flat on the floor and one toe is on the floor. This will enable a wide base of support and allow the arms to be INSIDE the legs in most cases.
Look at the load in order to grasp in properly but lift your head up before you take the weight. Gently rock the load before lifting in order to assess it's weight if you don't know what it is.