A simple macro will help with a lot of your healing. Set up something like this:

/assist Tank
/cast [help] Heal Spell; [help, target=targettarget] Heal Spell; [target=player] Heal Spell

Set this up for your most frequently used heals. Pressing the macro will target your tank's target and cast the heal spell if that is friendly. If the tank is targeting a hostile, then the first argument comes back as not valid and it attempts the second argument which is true if the target of the tank's target (almost always the tank or a dps that over nuked) is friendly. If that fails, then the spell heals the caster/healer.

You don't have to mess around with party1, party2 etc, as the heal goes to the target of the mob your tank (or your dps) is attacking.

Hopefully that helps.