Spell rotation:
Is VT ticking? If no, cast.
Is SWP ticking? If no, cast.
Is Mind Blast on cooldown? If no, cast.
Is SWD on cooldown? If no, cast.
Have I done all the above? If yes, fill in with Mind Flay.
Someone actually took the time to figure out a "rotation" for spriests, and the rotation was over 200 seconds long. It's a matter of keepign spells up or on cooldown and filling in with Flay.
Solo, you really need to feel for yourself. My level of +dmg is enough that VT->Blast->Flay->Flay will take them down to one or two wand shots (the demons in Ogri'la being the only real exception). I used to include SWP, but found it was pretty much a waste of mana as my +dmg got up. If I'm wanting to grind quickly against lower-level guys, I can often switch the Flay for Death and still grind near-endlessly (my current record being 3 hours non-stop in Skettis, I got bored and quit).