I'm curious as to which soul magic spells you guys think are best for PvP, and how do you obtain them? I'm a full blown sorcerer and I'm looking to do a fair amount of PvP but find my lack of offensive spells that aren't easy to dodge disturbing.

Strong magic shield is the only soul spell worth a damn. They're all too slow and easily blocked/dodged to be viable for pvp.

how viable soul magic is for pvp depends how honorable you want to be.
I've had some great luck waiting untill people engage enemies and then blowing them up in the back with some slow ass heavy spells. If you join the DarkWraiths you can even get a stone that lets you invade people without them being notified you're in their game. 

It's dark souls, being an asshole to opponents is with the spirit of the game.
 
If you don't want to be a huge asshole , then your best bet for pvp is to join the darkmoon blade, you can have 2 darkmoon invaders at once, and maybe you can land some of your spells if they're distracted with your buddy?

@Ulong: Are you sure that's DarkWraith? I thought it was Darkblade, I hope it's darkblade anyway because that's the whole reason I joined them; so that I could gank unsuspecting people. I've invaded a few worlds after joining them so far, and yah a few of the spells like Soul Spear have been real nice for hitting them when they don't see it coming. However when they do see it coming I seem to be boned unless the other player is really clueless as to how to dodge spells. I guess I'll be leaving the PvP aspect of the game to my warrior.

You know what though, I could even be wrong about everything I've said. I think the biggest problem with Demon/Dark souls is they weren't sure which information was fun to obfusticate, and which was just annoying to obfusticate.

I have no idea on the specifics really, if the stone does tell the player then that might explain a few invasions I made earlier where it seemed like the player knew I was coming.. kind of sucked that there were a couple of phantoms with the guy as well lol.

I remember in Demon's Souls one guy totally owned me by running up to me and casting Firestorm right outside of my melee range. Took me completely off guard and one-shotted me too.

This happened more than once too!

@Ulong: I've looked online and a cracked red eye orb warns people, a normal red eye orb doesn't warm them and same with the Darkmoon ring. But that puts you in peoples worlds when you are needed.