For the less skilled players (who aren't verbally abusive) I usually find it easier to adapt to their behavior. Taking a few examples from this thread:
Players that pull mobs separate from the tank: I usually let their HP drop to critical, then aim a healing circle at them. The idea is that they hear the low HP earning and flash, realize "hey, maybe I shouldn't do this", and follow the tank next time.
Players that run in circles: I'm usually moving when I aim focus heal, so I sweep my crosshairs in the opposite direction that they are running. This will catch them in the lock on. After that, I'll turn my camera to lock the tank and fire the heal. Focus heal is multidirectional so these players are not a problem.
Players running out of range: I usually focus heal people before they have the chance to do this, but if they are so far away that even focus heal can't lock on, then they are probably safe from the boss as well. Thus, I ignore the player until he gets back in range (on the radar) and quickly focus heal lock him when he gets into range.
People that dodge heals: In general, I find this happens the most when I mistime a heal. If a DPS has to choose between gambling whether my heal or the boss will hit them first, they'll use their next action to jump away. If an archer or sorc us in melee range of a boss, they will probably be jumping out at first opportunity as well. Regardless, I use circle heal only the melee is well positioned and unlikely to make a sudden evade; most of the time this means getting close to the boss, which is a-okay.