There are two ways I know of.
1. Respawn the Rift Enemies - aka The Brave, Brave Sir Robin Technique:
Find a rift with a particular enemy (Revenant, Arcane Horror, Rage Demon) in the opening spawn and focus kill it quickly. Loot the dead enemy, and then Run Away! Run Away! and run far enough away from the rift to get out of combat. The rift will close, and you can heal up. When you get near it again the same enemy should respawn for you to kill again. I used this method at a rift in the north east of Crestwood. An Arcane Horror spawns in the first rift fight. I usually stealth in a rogue behind it, have a mage Winter's Grasp it, then my rogue will twin fang / death blow it for s shatter and a quick kill. Looting it gets you an Arcane Horror heart and often some cloth.
2. Farm the Closed Rift - aka The Five Fast Fingers of Death Technique:
After you close a rift you get a rift loot drop. It usually contains an essence and another item. Take the essence and leave the other item. When you reenter the zone the loot drop will reapawn, usually with another essence. The rift in the south west of the Hinterlands drops fire essence and often a nice amount of high armor metal like 20 lazurite. Using this method requires you to remember where the rift was located.