After your first application of SW:P, unless you're occupied with an add or something, you should never have to hit the button again in the fight. One of the shadow talents makes it so that Mind Flay reapplies SW:P, so as long as you have Mind Flay in your rotation (which you should) SW:P will take care of itself.

Priority system, as I understand it, is SW:P and Devouring Plague should always be on your target non-stop. Vamp Touch too, but with a slight difference. Because you get extra damage when it's dispelled or falls off your target, you want to just barely let it disappear, ideally putting it on again a split second after it falls off.
Other than those DoTs, it's Mind Blast when it's off CD (unless you have 4-set t10, then you probably won't use it much at all) and Mind Flay when there's nothing else to do. One more thing about SW:P, since it gets automatically reapplied with the same strength as it was originally applied at, make sure you originally apply it with as many damage buffs as possible. Get 5 stacks of Shadow Weaving up, if you use anything like Muradin's Spyglass get it to 10 stacks before applying, anything you can do to max the damage that DoT will do, because that maxing will help you throughout the fight.

Vampiric Touch, Mind Blast, Devouring Plague, Mind Flay, Mind Flay, (5 stacks), Shadow Word Pain, you should be able to get another mind flay or two in after this, then refresh VT, MB, DP, MF, MF, MF.

Now one thing I have noticed is that this is correct for the big fights. However I am seeing that in a group of well geared 80's bosses with 1000K HP's don't really last long enough to see any major benefits from waiting for 5 stacks. Your best bet is to go to the practice dummies to see what gives you the best total damage for a 60 second fight.

Try this rotation. VT, SWP, MB, DP. MF, MF, MF, refresh rotation as VT is just falling off but skip SWP. See what kind of numbers you put up for a 60 second fight. Now compare the numbers to the first rotation for the same length of time. I think you will see a difference in the total damage.

And biggest rule of them all. Don't clip your mind flay.