How do you maintain the temperature of the milk over a period of time?

Create a water bath or bain-marie;

1. Place your pot of milk in a water bath in the sink.
2. Fill the sink with water which is the same temperature, or on cooler days, slightly warmer than the milk in the  pot.
3. Every hour, check the temperature of the water. Add more warm water if necessary during the day to maintain the temperature. This will provide you with an insulating blanket to keep the milk at the desired temperature.

Note: Don't be tempted to surround it with boiling water to make it last longer, the temperature could be too high for the culture and could kill it.

Oven;

1. Leave in a warm oven with the light left on to seal in warmth.
2. Check on it to ensure it doesn't go below temperature, but be careful not to open the door too often and let out the heat.

Hot Water Cupboard; 

Store in your hot water cupboard if you have access to this in your house.