I can't stess this enough: 
  
"Another important tip I'd give Warlock players is to skill your First Aid skill. Warlocks have the benefit of Life Tap to turn hp into mana and bandaging up heals you faster than sitting down for food or drink. This reduces downtime between mobs." 
  
When in a group just spam life tap and tell your healer only to heal you if he can spare the mana and then use First Aid when he doesn't heal you.