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Cognitive and sensorimotor processes interact during the practice of a motor skill, with consequences on motor performance, memory, and transfer of learning. Some experiments show that cognitive effort (incited by the conditions of practice or by the possibility given to the learner to control some parameters of the situation) improves motor skill learning. However, conscious control, with the focus of attention allocated to task execution, has detrimental effects on motor performance and learning, supporting a dualistic point of view considering a functional dissociation between a cognitive mind and a sensorimotor body. The present paper presents some experimental works dealing with these topics and suggests that the level of task difficulty (related to the skill level or the motor deficiencies of the learner) is an important factor modulating the effects of the condition of practice. When the level of task difficulty and the conditions of practice can be chosen by the learner, motor skill learning is improved. These issues must be taken into account for the setting of practice conditions for motor learning or rehabilitation.