Regarding action bar use, I use primarily the buttons 1-4 for attacks by warlocks and my mage while leaving these blank for my priest.

These four buttons are needed for direct single target damage, dots, drains and AoE. I use [modifier:alt] and [nomodifier:alt] to get a second function on the buttons, such as shadow burn and fire blast instead of shadow bolt and fire bolt or drain mana instead of drain life.

I have previously used smite on the priest as well, but stopped doing that to be free to heal whenever needed and to have mana reserves when needed.