Timber floors can be insulated by lifting the floorboards and laying mineral wool insulation supported by netting between the joists.
You can also use a regular tube sealant, such as silicon to fill gaps between floorboards and skirting boards to stop draughts. A note of warning, however - don't block under-floor airbricks in your outside walls. Floorboards will rot without adequate ventilation.
For more on how floor insulation works, costs, savings and other considerations see the Energy Saving Trust information page.