Most modern conceptions of volition address it as a process of conscious action control which becomes automatized (e.g. see Heckhausen and Kuhl; Gollwitzer; Boekaerts and Corno).
[1] Kurt Lewin argues that motivation and volition are one and the same, in distinction to the nineteenth century psychologist Narziß Ach.
In the book A Bias for Action,[2] Heinrich Bruch and Sumantra Ghoshal also differentiate volition (willpower) from motivation.
Using this model, they propose assessing individuals' differing levels of commitment with regard to tasks by measuring it on a scale of intent from motivation(an emotion) to volition (a decision).
Discussions of impulse control (e.g., Kuhl and Heckhausen) and education (e.g., Corno), also make the motivation-volition distinction.