I use Shift + a mouse key on the side of my mouse to "Target target's target".
I use the same key without Shift to turn cleric stance on / off.

This is extremely useful as a WHM, because it allows me to immediately begin attacking whatever the tank is attacking. Also, as long as the tank has hate, pressing the key again will switch me from the mob back to the tank. Pressing the key without shift switches cleric stance, allowing me to heal / DPS. The mouse key is linked to a hotkey in the hotbar.

So I start with Shift + mouse key = target tank's target.
Mouse key = CS on
DPS DPS DPS
Shift + mouse key = target tank (because the target of the mob is the tank)
Mouse key = CS off
Heal
Rinse and repeat.

When I need to target other mobs or party members I use tab or F3, F4 respectively.