I use this macro to heal on my priest. I use a warlock as my main, and select healing targets using him if I am lazy. This macro will heal your main's target if it is friendly, or heal your mains targets target.

Just change Power Word: Shield out for your spell.


MACRO 16777224 "Sheild" INV_Misc_QuestionMark
#show Power Word: Shield
/stopcasting
/assist (*Name of your Main*)
/cast [target=targettarget,help] Power Word: Shield
/target (*Name of your Main*)
/cast [target=targettarget,help] Power Word: Shield
END