In a talk that apparently previews the argument of a much larger book(s) on the subject, John Barclay draws on ancient practices of gift-giving to argue that God's χάρις ("grace" or "gift"), according to Paul, is unobligated (so Luther) but not without obligation (contrast Luther). Just as in the ancient world gift-giving always set up a complex system of reciprocity--much like a covenant--so too with God's "grace": God's gift of grace is given to unworthy sinners who have no claim on God's mercy, but those who receive the gift are obligated to respond appropriately.
This is not sunergism, it is not completing grace with works, or making subjectively true what is objectively true--because the gift changes everything. The gifts that is grace is life--a transfer of realms from the rule of sin and death to the realm of life--and anything we do is made possible in and through the resurrected Messiah. (Another way of saying this is that it is the Spirit that gives life.)
The Christian, then, is not simul iustus et peccator ("simultaneously righteous and a sinner")--to use Luther's language--but simul dead and alive. The body--dead because of sin but alive because of righteousness--is the site of a battle. Those who are made alive must experience mind-renewal, which is always exhibited in a transformation, a change in habitus, from one set of bodily practices to another. This is no individual struggle because the body, for Paul, is corporate, and the disciplines of the Christian life are lived out in community.
Note: I jotted down these notes on paper within a week or so of listening to Barclay's talk on a drive to Regina last summer. Since I can't vouch for their accuracy, I recommend listening to the lecture (online here), reading the conference proceedings (just released by Baylor University Press), or, if you are a visual learner, watching this video on a related topic.