I focus my main and use /cast [target=focus-target] DPS for my damage and /cast [target=focus-target-target] Heal for my healing. It makes it so you heal whoever has aggro (in case an alt gets aggro instead of tank). You can also put in a /cast [focus-target, help] Heal in first, so incase you want to heal a friend outside of your group (heals whoever the focus is targeting).