Originally Posted by Moriat
/cast [@party1] Power Word: Shield
The flash heal / PoM one:
/castsequence [@party1] Flash Heal, Prayer of Mending
Note: If you are doing lots of BGs with your arena partner, exchange "arena1" with his/her name, then it will work in BGs, and any other place, as well.
For BGs I would rather recommend you mouseover macros, simply hover your mouse over the one you want to shield (on either the raid frames or "in person" or whichever spell and click your binding.
/cast [@mouseover,help] Power Word: Shield
Hope it helps.