Depending on how bad your knees are, you may also use the lotus or half-lotus pose to stretch the outside of the knee. The pose is actually designed to strech your hips, so make sure you don't put too much pressure on your knees as this can cause damage. To stretch the inside of the knee, the pose is called "Turtle Pose" and is like kneeling, except you spread your knees out as wide as possible and your toes meet behind you. Try to put your butt on the floor (I can't). Again, be careful with your knees.
-FJ