"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)
Demon Hunters normally have a maximum of 125 Hatred, which regenerates at the rate of 5 per second. While no items grant an increase to maximum Hatred, affixes on Cloaks, Quivers, and Hand Crossbows can add +6 to Hatred regeneration. Furthermore, with the Archery passive, dual-wielding Hand Crossbows increases Hatred regeneration by +1 per second. The Templar can add +1 Hatred per second, as can the Bat Companion (the Bat can also restore 50 Hatred at once). Primary skills (as well as Chakram and Elemental Arrow, in certain cases), generate 4–7 Hatred per damage-causing attack; this is increased by +2 by the two-piece bonus of the Unhallowed Essence set. The Punishment rune of Preparation adds 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)
Demon Hunters start with a base maximum of 30 Discipline, which returns at the rate of 1 point per second. 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.