Even if your attic is well insulated, the folding stairs that provides access can be a major source of heat loss. They often are poorly sealed and provide only a thin piece of plywood to keep the heated air from escaping.
Begin by checking to see if the attic stairs close tightly, then apply self-adhesive foam weather stripping around the perimeter where the plywood door meets the frame.
For extra protection build an air trap above the stairs using rigid Styrofoam insulating board. Tape together a frame around the four sides, tall enough to accommodate the folded stairs, and then hinge a door on one side using the same tape. The whole unit is sealed around the perimeter of the entryway with more tape, creating an insulated air trap between itself and the attic stairs.
Read our article on How to Insulate Attic Stairs to find out more.