Second only to insulating the attic, insulating basement and foundation walls is often a cost-effective home improvement. It's one that can also make the home more comfortable.
Basements that are used as living space and heated or cooled should have insulation separating the living area from the outside air or ground. If there is none, you can build a wall frame similar to the above ground walls, attach it to the basement wall, add insulating batts between the studs, and then apply wall board over it all to give it a finished look. Just be sure to use vapor barriers as recommended for your area. In most parts of the country, the vapor barrier should face the living area. And check for moisture or drainage problems before covering an area with walling.
Batts can be inserted between the floor joists and secured with flexible metal tongs or laced in place with wire wrapped around nails or held up with chicken wire. Make sure vapor barriers are used appropriately. In all but the most humid parts of the country they should face up or toward the living space.
Also, insulate the rim joist, the area on top of the foundation where floor joists abut the rim joist. Fiberglass batts cut to fit snugly or pieces of rigid insulation can be fitted into the rim joist area. Check local recommendations on vapor barriers. In most climates they should be installed facing toward the living space, so under the house, faced batts would have their facing side up and the unfaced side exposed.