So, you've got your ceiling insulated, and are wondering if you should insulate under your floor also? In terms of heat loss, carpeted floors account for approximately 15%, but raised floors with exposed floorboards lose about 22% of heat. This figure increases to approximately 30% if you also factor in air draughts in floorboards. Given than ceiling heat loss is 35%, you should strongly consider underfloor insulation if you have exposed floorboards on a suspended wooden floor.
Underfloor insulation is typically done by using R2.0 Glasswool batts that are placed securely between floor joists, and then supported with either special staples or sometimes cement sheeting in high wind areas.
Underfloor insulation will increase the ambient temperature of the house by preventing air draughts between floor boards, and also by creating an insulating air pocket under the floor boards, helping to trap heat in the house. The floorboards will essentially feel like room temperature AND stop air draughts - a double bonus.
Not all houses are able to have underfloor insulation. Houses with suspended wooden floors with adequate crawl space are able to have their underfloors insulated. Houses with bare wooden boards or only thin coverings would benefit more from underfloor insulation, especially if the wind is able to blow under the dwelling (which saps the heat away from the floorboards).
Floor coverings, such as thick carpet with underlay, provide a degree of floor-based insulation inherently, but will still benefit from a thick layer of insulation sub-floor (about 10 times thicker than underlay and carpet combined thickness).