Starting in 1776 and continuing through the end (!) of the Civil War, there was a gradual evolution in the institution of slavery in the US. Some of the changes could possibly be described as making slavery more "humane," although that's not necessarily the word I'd use.
Slave marriages were not legally recognized, and were initially discouraged. But when the slave trade was outlawed in 1807 it became important to slave owners that their slaves produce enough children to keep up the population. Marriage was also seen as a way to keep the slaves from becoming unhappy enough to revolt. Possibly for reasons of economic organization, northern slaveowners never participated in this (see http://www.pbs.org/wnet/slavery/experience/family/history.html ). People who owned small numbers of slaves often continued to break up families by selling their members, or arranged marriages between men and women who worked in different places, so that the couples could only visit intermittently.
In some ways the institution may have become less humane. In the colonial period, slaves were often allowed to learn to read, and were sometimes even encouraged to do so (so that they could read the Bible), but were usually forbidden to learn to write. However, after Nat Turner's revolt in 1831 there was a wave of legislation imposing more strict limits on the education of slaves.
In some respects, slaveholders in the late antebellum period felt obliged to offer more detailed justifications for slavery, without fundamentally changing the underlying institution. Biblical precedents for slavery were described in greater and greater detail. Tracts defending slavery were eagerly consumed. Economic justifications such as the mudsill theory were offered, as well as rationales involving a system of mutual obligation.
Surprisingly, there was a strong current to reform slavery from within the Confederacy, led by clergymen such as Nathaniel Macon Crawford and Henry Holcombe Tucker. Boles (Masters and Slaves in the House of the Lord: Race and Religion in the American South, 1740-1870) argues that they may have acted partly out of a desire to strengthen their social system against attack from the North and partly because the war freed them to argue for reform without being accused of disloyalty:
Beginning late in 1862, reform churchmen launched a determined campaign to rectify slavery's most glaring ethical shortcomings. [...] clergymen urged lawmakers to legalize black education, allow bondsmen to preach the gospel, protect slave marriages, and prohibit separation of black family members.
