For heals I have a button for each party member.
Usually F1 - F5

I like to keep fighting after the heal so this is how I have mine set up. I also use this method for any other quick buffs that need to be cast during combat.
Note1: I actually change party1-5 to the names to make sure I get the right one.
Note2: You may not need the entire /assist line depending on the character's order in the party that will be casting the heal.

Code:
#Showtooltip!
/stopcasting
/target <party_member_to_heal>
/cast <heal spell>
/assist [target=party1, nodead, help, exists]; [target=party2, nodead, help, exists]; [target=party3, nodead, help, exists]; [target=party4, nodead, help, exists]; [target=party1, exists]