Limit winter heat loss by carefully insulating your greenhouse. However, bear in mind that savings on heating costs are at the expense of reduced light levels, reduced ventilation and higher humidity. Insulating the side facing the prevailing winds is a compromise which saves costs with little loss of light and retains good ventilation.
Before cold weather sets in check the greenhouse for cracks and broken panes of glass as some heat will be lost through leakage. Repair badly fitting doors and vents. Some ventilation will be needed to counter condensation so do not seal over vents completely. If using gas or paraffin heaters - which produce water vapour - you will have to ensure some form of ventilation or rots may become a problem. If there are no plants growing in the ground cover the soil with paving, gravel or sheeting to insulate the floor.
If some plants need no additional warmth, partition the greenhouse to keep the warmth insulating one end with plastic or thermal screening materials. Keep the most tender plants in this area.
Plastic
Bubble plastic - especially triple-walled - insulates better than clear plastic, although it will cut out more light. Attach plastic to the framework rather than direct to the glass as a gap between the glass and plastic will create an air pocket improving the insulation properties. Remove bubble plastic in spring, when light is needed for seedlings.
Thermal screens
Trap warm air in the lower half of the greenhouse at night by suspending at head height clear plastic or a purpose-made thermal screen material with strips of aluminium tape that deflect heat back into the greenhouse.
Side panels
Clad the lower portions of the greenhouse in winter below the staging with wood, polystyrene or rigid plastic panels.