I've been writing the shadow version of Spiritual Guidance for over a year and a half now. In actuality, it seems like forever -- like I've covered every topic multiple times. And yet, in all those columns, there's one topic I've never really taken the time to address: shadow priest PVP.
There's a reason, of course. PVP in and of itself is a damn complex subject. With a PVE encounter, there's some randomness, but it's all contained within a predetermined set of variables that you can research on a resource such as Wowhead. You may not know when Onyxia is gonna use her Deep Breath ability, but you know it's coming.
PVP is different. You don't know what your opponent is thinking (if he's even thinking at all). Some battleground teams head into battle looking for a street fight, hoping to win with raw brutality and no real strategy. Other teams choreograph their movements and communicate through Ventrillo or Mumble, approaching each battle like amateur Douglas MacArthurs. There's no algorithm that tells you how your opponents will act and react to what you do.
I'm far from a PVP expert myself, so I can really only offer you some of the PVP basics to get you started. If you're looking for top-level strategy, check out Blood Sport. If you're looking to get your feet wet and start exploring the other side of the World of Warcraft game ... read on.