How do I maintain my milk at the appropriate temperature for long periods of time?

A kitchen sink is the best choice for this. The easiest way to keep a pot or vat of milk up to temperature is to put it into a sink full of hot water. As the milk begins to warm to target temperature and the bath begins to cool, drain some water from the sink and add some of your boiling water to keep the differential at about 10-15 degrees. Then, as you get within 7-8 degrees of your target, reduce this more. Always keep the final bath 2-3 degrees above the milk temperature.