Creating incentives for slaves

In colonial Virginia there were indentured servants, who served their master for a fixed term, and slaves who served for their lifetime. For example, in return for paying someone's travel to the New World they would agree to provide labor for a set number of years. By contrast, slaves were not parties to their sale.

But how did you motivate indentured servants or slaves to work? In the case of indentured servants, their term of service could be extended. That threat stood out as an incentive to work.

By definition, extension of service does not apply to a slave who serves for life.

Here's how the law of Virginia solved that problem (source):

OCTOBER 1669; IN THE TWENTIE FIRST YEARE OF THE RAIGNE OF OUR SOVERAIGNE LORD KING CHARLES THE SECOND. ACT I. An act about the casuall killing of slaves.	Edi. 1733 and 1752.
WHEREAS the only law in force for the punishment of refractory servants (a) resisting their master, mistris or overseer cannot be inflicted upon negroes, nor the obstinacy of many of them by other then violent meanes supprest, Be it enacted and declared by this grand assembly, if any slave resist his master (or other by his masters order correcting him) and by the extremity of the correction should chance to die, that his death shall not be accompted ffelony, but the master (or that other person appointed by the master to punish him) be acquit from molestation, since it cannot be presumed that prepensed malice (which alone makes murther ffelony) should induce any man to destroy his owne estate.