I have a bit of a different slant on PvP play for shammys. Before I get into that though, I want to mention two caveats:
1) My shaman is a lvl 38 twink, so my advice is based on and applies best to playing that level. Others' recommendations and experience in the higher lvls should take precedence over what's written here when the two contradict. Similarly, I basically stick to the BGs. While I can't see why this would not apply to arena play as well, that is not the laboratory I use to put my shammy through his paces.
2) As my twink, I have geared him to the teeth:
Ardent Custodian with
Crusader enchant,
The Green Tower,
Gloves of Holy Might, etc... (you get the idea). I mention this because certainly some of the toon's effectiveness is linked to top of the line gear. I don't think that this gear (or any gear, for that matter) makes one a better player. However I do think it gives one more room to make a mistake now and again and not always get creamed for it. (OK, when I stand toe-to-toe with a warrior and outslug him, that's the gear.)
With that out of the way, I want to profess that I LOVE LOVE LOVE playing PvP with my shaman. The class's versatility not only makes it ideal for handling every situation, but also a heck of a lot of fun. I can melee with the best of them, hang out on the side of a skirmish and pick off unsuspecting enemies with shocks, LB and CL, or just chill in the back healing, providing teammates with near-unlimited health while letting them do the hard work.
Which leads me to my somewhat alternative view - I am mostly in Resto and wouldn't have it any other way. (The rest is Enhance, with zero in Elemental.)
As I see it, with PvP there are two concerns - killing the enemy and staying alive, not necessarily in that order. As a shammy you need to use both, and can do both at once better than any other class, IMO. When facing a melee class like rogues and warriors, I know I am looking to outlast them. I will stay alive long enough that even with their superior DPS they eventually run out of health. On the other hand, when facing a caster I am primarily looking to bash them before they link together some crits and blast me. So there I am focusing on the damage aspect first, but still conscious of my superior ability to muster HP. Pallys are tricker and require some of both, plus ideally some ranged attack.
To create this endurance shammy, I want talent points to do most of the work. With
Rank 5 Healing Focus, a great majority of my heals go uninterrupted. I can sit and spam Lesser HW and there ain't too much someone can do about it. With some other helpful Resto talents and a healing-friendly armor item or two, I can last a long, long time.
In any situation my ace in the hole is
Nature's Swiftness. What a thing of beauty this talent is. When outlasting an opponent, tossing in an instant HW among the timed & [mostly] uninterrupted ones is darn near like starting with full load of HP while your opponent can only stand by and watch. (Again, pallys have a similar ability so it's not as clear cut with them.) Similarly NS can be used with LB and CL to generate some quick damage at the beginning of a fight. A personal favorite of mine is to get in to range of an unsuspecting foe, hit with a CL, then an instant NS/LB combo, and immediately follow with a shock (also instant). Quite a way to soften up your opponent, eh? [Note: Offensively I use NS mostly with LB, which is the slowest spell to cast.]
However you must consider that, at this lvl, reaching NS has to be a planned objective. It takes a bunch of points to reach this talent, but it is oh so worth it. I don't know if I will lvl this toon up and if so what changes that will bring, although for now this set up is working very well for me.
Hope this helps.