Your mattress should be quite hard - not sagging in the middle. If it is, use a board underneath, or better - get a new one.
Side posture with flexed knees, which decreases the low back curve. The pillow should support the neck.
If a cushion is placed under the knees in this position, the low back curve is corrected.
To change the habit of sleeping face down, raise the foot of the bed as shown below.
Arrange your pillows correctly for reading and resting on the bed.
This position accentuates the low back curve, if the mattress is too hard.
A pillow which is too high overstretches the neck, arms and shoulders.
Facing downwards tires the neck and shoulder muscles.
Even if you bend the knee and hip, the lumbar curve is still accentuated when face down.