I just focus target the boss so I can switch to it easily if need be (along with seeing what its casting) and use party frames to target friendlies with my mouse. In the case of mob packs I either tab target, or if I've already healed the tank, then it brings up all of the mobs in the threat window which lets me easily click on the lowest one there. I've never really had issues with switching targets frequently. It's just part of the job and something you get used to.