Pay attention to your posture though. This is not "standing at attention," as locking your knees is bad news (as stated). However, keeping your core engaged, your lower back relaxed, your tailbone dropped, and your shoulders and chest broad, relaxed, and open, will take some pressure off of your knees.
If you can, keep moving. Walk around, even if it's just in a little area; do some kneebends, stretch your hip flexors, keep the joints moving.