"Demon hunters are perpetually at war with themselves. They hold no hope for a return to their former lives, but they fight and stalk demons with a passion. Those hunters who live to train others become deliberate and cautious and, yet, their impulses scream to meet demonic violence with more violence. Every arrow that demon hunters launch is dipped in spite; nearly all of their attacks are delivered with Hatred, a cold, righteous anger that renders them utterly merciless to their enemies. Their Hatred is nearly endless; it recovers quickly, as, even at rest, demon hunters are filled with the desire for vengeance."— Game Guide(src)
Special magical affixes on Cloaks, Quivers, and Hand Crossbows can increase the rate of Hatred regeneration, which normally each Demon Hunter has a maximum of 125, regenerated at the rate of 5 per second, but they may gain up to +6 per second from items. In addition, Blood Vengeance skill adds another +25 maximum Hatred, and Bat Companion and Templar both add +1 Hatred per second (Bat also can restore 50 Hatred at once). With Archery, one will gain +1 per second if dual-wielding Hand Crossbows. Paragon levels may grant +25 more Hatred. Primary skills (as well as Chakram and Elemental Arrow with certain synergies) generate Hatred on damaging attack, up to 7 per cast, and additional +2 with Unhallowed Essence set. Punishment is a rune choice that recovers 75 Hatred instantly.
"Those hunters who would hope to make any sort of true impact must temper their Hatred with a second resource: Discipline, the caution and care that ensures they live to kill another day. Discipline is precious to demon hunters. Not only does it return slowly, requiring more reflection and patience than their Hatred-based attacks, but they must rely on it for every defensive maneuver — trapping enemies in place, vaulting out of the way of teeth and claws, and dodging hails of spines and arrows. It is a simple matter for demon hunters to stand in place, icy with Hatred, and send a volley of projectiles at the hordes of their enemies. But, when they fail to defend themselves with Discipline, hunters leave themselves able to be slaughtered. A careful balance of Discipline and Hatred is the most likely way hunters ensure their survival."— Game Guide(src)
Discipline returns at the rate of 1 point per second, and Demon Hunters possess only 30 total Discipline without equipment. Currently, there are no ways to increase Discipline regeneration, though many skills allow generating it from hits and Critical Hits. However, Demon Hunters may gain up to +36 maximum Discipline from items (+46 with legendary items, namely bows, quivers, and some rings, most giving +9-12 discipline), and +20 from Preparation (Invigoration rune), which is also a primary way of mid-combat Discipline recovery. Perfectionist skill also reduces cost of Discipline skills by 10%, rounding down. Bitter Pill rune choice allows some chance on hit to regain 2 Discipline. Maintaining Discipline is especially important for the Unhallowed Essence set, as the 6-piece damage bonus is proportional to Discipline.