There are many long term plans you can put into place but we find most of the time we are never prepared for the worst.
The immediate things to do would be to cover your tank in an insulated material, blanket etc (not anything with lose fibres like loft insulation)
Make sure the room your tank is in is kept as warm as possible with windows closed etc. Polystyrene is also a god material for insulating the tank, so if you can but this on the sides and front of your tank it will slow down the loss of heat. Smaller tanks lose heat faster than large tanks due to the smaller volume of water holding the heat.
Make sure when the power is back on that you don’t let your tank warm up to fast as the fish will be effected.