Site selection is quite important. Stay out of creek bottoms and places where cold (heavy) air will settle. That is easily worth about 5-10F right there.

Do not sleep on cots. Get down on the ground and insulate, as others have noted, with two or so pads (not air mattresses!).

I like to wear a warm base layer inside the bag,with something that covers my head and the large blood vessels in the neck.

Have a bit to eat before retiring. Chocolate is great for this purpose.

I wonder if your sleeping bags were honestly rated. Believe it or not, some manufacturers will exaggerate. 20F isn't really all that cold.