I wrote this tip for a student who was having difficulty with her knees in yoga. But the tip works great for sitting, climbing stairs and many other knee-bending activities.
When you begin to bend your knees in a yoga pose like Utkatasana (Chair), or Virabradasana 2 (Warrior), take a moment to change your thinking. The habit that we all have when we bend is to think about lowering ourselves down in space. We end up pressing down into our knees.
Instead, imagine your whole body moving up as you bend your knee. To avoid pushing down into the knee joint, imagine the knee swinging away from the hip and ankle joints. Thinking head and body up, and knees forward and away will leave your knees feeling more spacious.
Body Project Blog: Where Thought is the Active Ingredient.