1. Bubble wrap
Using bubble wrap can help to unsulate your greenhouse. Attach the bubble wrap to the inside of your greenhouse to create a 'double-glazed' effect.
To protect even further, you can tape down each joining sheet to stop draughts.
2. Greenhouse heaters
Paraffin heaters or electric heaters (if you have an electricity supply) are a good way of keeping the temperature up within your greenhouse.
To check that the heating level is correct in your greenhouse, use a thermometer.