From a great guide to texturing milk on coffeegeek - CoffeeGeek - Hello Milk!
"The above is a complicated way of explaining where the old adage that you can “foam milk once and steam it twice” comes from. To foam milk a second time won’t work because the proteins that facilitated the formation of the foam initially have become denatured. You can of course just reheat the milk a second time but even that is not recommended. Steam only as much milk as you need for a given drink. Start with cold fresh milk every time. Never resteam milk or add fresh milk to already steamed milk. It is considered poor form. "