I run my disc priest with a fall-through style mouseover setup.
Basically:
/cast [@mouseover,exists,help] spell
/cast [@target,exists,help spell
/cast [@targettarget,exists,help] spell
/cast [@player,exists,help] spell

I have most of my heals on my mouse (mx1000) with the exception of a few. This way I can just leave my priest on FTL Assist and he heals whoever is being attacked by my DPS targets and in a pinch I can mouseover for spot heals.