My opening rotation goes like this: Volcanic Potion, Shadow Word: Pain, Shadow Fiend, Mind Flay x 1.1
, Mind Blast, Vampiric Touch, Devouring Plague, Archangel. From there I just start Mind Flaying and going into the normal rotation, refreshing dots whenever a stronger buff becomes available.
I'm aware that there are a lot of different opinions on opening rotations. Many feel it is best to lead off with dots first, and then build buffs later. The reason I do not advocate this method is because at the start of the pull, you have a lot of procs going off - shadow orbs, trinkets, enchants, etc... If you lead off with dots, they are ticking at their lowest settings, and will need to be refreshed as soon as buffs become active.
The other advantage of this rotation is that it advocates using Archangel early off, and while a lot of intellect buffs are still active. If you get a bit of luck, an orb will proc before the Mind Blast, then your dots go up at their strongest setting and do not need to be refreshed, and you are going right into Archangel with your intellect procs still active. It's a gamble, yes, but if it works it pays off with a very large burst at the start, and that's the reason I advocate it.
(I have tested the above opening in Simulationcraft, and it appears to hold up and perform soundly.)