My healing is based on who is getting beat on.
/cast [target=focus-target-target] HealSpell
It heals whoever the main's (focus) mob (target) is beating on (target). If you get fancy you can add more.
/cast [target=focus-target, help] HealSpell
It heals whoever the main is targeting if it is helpful (if you can help it). I put it first so I can heal people in an emergency and heal outside my group.

Hope that helps. Good luck.