It depends how the instigators of the �perfect race� intend to do it. In terms of the Nazi�s it was a social philosophy (or indeed, policy) through selective breeding. Hitler had a penchant for young strapping blonde boys in short lederhosen and thus selected them to be the fore-bearers of the future master-race. This is eugenics, as said above, although they also used genocide to filter out what they considered to be undesirables. Eugenics can be by race or class.
Another way of creating a �perfect race� could be through reprogenetics which is the clever word that clever people use, when Channel 4 or Channel 5 use �Designer Babies� or �Embryo Selection�.