I'll usually mark 1 - the priority/biggest threat. For trash mobs I'll swap off when the main target is at like 10% to let everyone know the next target. We've seemed to get by so far. If I notice a different mobs hp dropping, I figure a dps is off and I'll switch.

What macros do people use for marking? I still have an issue trying to tab through enemies and having the focus jump across the room.