I will say one thing that helps is to ask the party if they want you to sleep things. This often works as a cue for other DPS to focus on a single target. Also, if the tank insists on switching between multiple targets or using Overpower, let him know this little fact: When a mob is asleep, the healer won't get enmity for healing the tank. So, if he had hate on the mob before it was slept, he should have hate on it when it wakes up.