I've trained a lot of people who experience knee pain when doing simple movements. This one tip has literally changed their lives. When you're doing any movement the first joint you move takes a large portion of the load of your body and whatever weight you are holding. Most people don't realize they are starting a movement with the knees. Try slightly bending at the hip first then moving the knees after, remember this not just in the gym but your daily life (ex. walking up stairs).