Extensive theoretical, psychophysical, and neurobiological work has focused on the mechanisms by which short-term learning develops into long-term memory. Better understanding of these mechanisms may lead to the ability to improve the efficiency of training procedures. A key phenomenon in the formation of long-term memory is the effect of overlearning on retention - discovered by Ebbinghaus in 1885: when the initial training period in a task is prolonged even beyond what is necessary for good immediate recall, long-term retention improves. Although this overlearning effect has received considerable attention as a phenomenon in psychology research, the mechanisms governing this process are not well understood and the ability to predict the benefit conveyed by varying degrees of overlearning does not yet exist. Here we studied the relationship between the duration of an initial training period and the amount of retention 24 hours later for the adaptation of human reaching arm movements to a novel force environment. We show that in this motor adaptation task, the amount of long-term retention is predicted not by the overall performance level achieved during the training period, but rather by the level of a specific component process in a multi-rate model of short-term memory formation. These findings indicate that while multiple learning processes determine the ability to learn a motor adaptation, only one provides a gateway to long-term memory formation. Understanding the dynamics of this key learning process may allow for the rational design of training and rehabilitation paradigms that maximize the long-term benefit of each session.