The only guaranteed method is to either type the tank's name in the macro or set him as focus and use focus-based heal macros for him.

An easier method that works pretty well as long as the mob(s) has the tank targeted would be:

/cast [@targettarget, help] healingspell

That way, you can target the enemy and just heal the tank as needed with one button. If the mob switches targets, that tends to break down a bit but it's a pretty easy way to do it without having to do any special targeting or macro writing for every new group you're in.