I prefer to have my RL mark the person we need to think about as Tank or Assist using the raid frame tools already present in game. Thus I can reserve my focus targeting for something more important to the job I am best at - melting faces.
It's quite simple - have your RL right click the people in question and "set Assist" or "Set Tank" (or whatever its called), then you simply click and drag the icon out of the blizzard raid window and you have all of them right there. Alternatively, some addons do this automatically.