Sleeping on the left side with a pillow wedged between your legs and behind your back will help to increase the venous flow back to the heart. Because the inferior vena cava is on the right side, when you lay on the left side you relieve the stress on the vein by removing the weight of the uterus.
Consider the purchase of special support hose or graduated compression stockings. These will help to improve the blood flow back to the hearts and prevent swelling in the lower extremities. To help prevent blood from pooling in the legs, put these stockings on before getting out of bed in the morning and keep them on all day long. These stockings may be uncomfortable in hot weather but will save your legs in the long run.