If you’re playing a summoner build like Wraiths, where you summon a large number of fragile minions that aren’t expected to last very long, you can get pretty solid sustain from on-death effects like Mania of Mortality or the Death Rattle unique, because you constantly have minions dying to feed them. You can get extra mileage out of this by taking the Putrid Wraith passives in both Necromancer and Lich (neither requires unlocking the high-tier passives), so that you have even more minions dying nearby to feed your health pool.
That said, I’m not sure what you’re supposed to do if you focus on fewer, stronger minions like golems. If you don’t have minions dying all the time, it is indeed very hard to sustain, especially against small numbers of strong enemies where you can’t rely on on-death effects.
As I’ve said before, I think it’d be a good idea for Sacrifice to have a branch focused on eating minions for buffs/sustain, which might help with this. There’s also the fact that several of the Acolyte skills that could reasonably be expected to help with sustain (like Transplant and Soul Feast) don’t have trees yet. Maybe this problem will go away once those skills get fleshed out (the ward nodes on Mage’s Teleport certainly helped them out).