I don't change targets at all if I can help it (on my alts). I use /cast [target=focus-target] DPS for damage and /cast [target=focus-target-target] Heal for my heals.

I didn't always let me heal a friendly person cause they would be targeting a mob. So I put in a /cast [help] Heal in along with my normal heal, so in case I target a friendly, it works, it it is a mob, it goes to the next one (heal focus's target-target).