Focus targeting is essential un high-end PvP. The way we do it is one of two ways.
A) We assign a target caller (usually a sniper) and all the dps set him as the focus target. When someone from the enemy team gets low, is out of CD's, or is otherwise a priority, he will call a switch.
Our dps will use they acquire focus target's target keybind and promptly hard-switch on to the target and burn him down. If he pops a defensive CD or gets guard, deending on the CD and his hp, we might leave him alone, or continue dpsing.
B) In ranked the norm (not including Synergy...) is to run 4 dps. So we have two, two-man teams. We have them focus off each other, and they call separate targets. When someone gets low or needs to go down, we hard-switch on using all 4 dps for a kill.