There are too many rotations without any given reason. Let me explain:
SW:P = Shadow Word: Pain
DP = Devouring Plague
VT = Vampric Touch
MF = Mind Flay
AA = Arch Angel
MB = Mind Blast
SW = Shadow Word: Death
Firstly, my rotation on a boss is; SW:P, DP, VT, MF, MF, AA, MB -- Then whatever is to expire or comes off cooldown first in the priority of VT, MB, DP.
The reasons to start off with the instant cast dots is not that they do more damage, but because often you have to move into position to begin a fight. Secondly, SW:P dot ticks have a chance to give you a Shadow Orb. When you cast a Mind Blast with 1 - 3 Shadow Orbs, your shadow damage is increased depending on how much mastery you have.
Every time you need to move, use SW and DP to ensure you are constantly damaging the boss.
There is an addon called "FaceMelter" which advises on what should be used next. I use it as a guide, but I still use my own instinct to know what to use next. It's great if you didn't realise an ability fell off the boss. A lot of haste and Bloodlust especially can sometimes invalidate what should be cast next.