Because mobility is key in PvP, and shamans have very few options when it comes to this, Ghost Wolf is an important ability that affords them faster movement across the battlefield. Almost all shaman PvP builds will have points in Improved Ghost Wolf, making it instant cast. A few funny things to remember: Ghost Wolf is a magical buff and not a form, so it can be dispelled with abilities like Dispel Magic and Purge; it does not remove movement-impairing abilities in the same way druid forms can shake off snares and roots; shamans in Ghost Wolf form can still be Polymorphed; the shaman becomes classified as a beast, making her susceptible to Scare Beast and Sap. Ghost Wolf has plenty of drawbacks. Pick one and abuse it.
This is my favorite shaman ability and is arguably the best PvP ability they have. While shamans are pretty limited in self-dispels -- they can't do jack against Magic effects, the most common type of debuff -- they are extremely effective offensive dispellers. Expect a shaman to begin their assault with a barrage of Purges in order to strip you off buffs and shields. Even restoration shamans are in the habit of using Purge often as it helps their teammates greatly. There isn't much you can do about this other than to pay attention and be prepared to recast important buffs such as Power Word: Shield at strategic moments.
Bloodlust / Heroism
Although this is on a relatively long, 5-minute cooldown, Bloodlust or Heroism can give the shaman an edge in the battle. Here's a simple tip: it's a magical buff. It can be dispelled just like any buff, or even better, stolen. If you don't possess offensive dispels, you'll be in a slight pickle considering it lasts 40 seconds. One quick note to remember against offensive-minded shamans (elemental or enhancement) is that the buff adds nothing to their defensive capabilities. Counter Bloodlust or Heroism with an all-out offensive yourself. Things change in team scenarios, obviously, as a whole bunch of enemies with the buff easily spells a quick death.
One of the shaman's best defensive spells, a Grounding Totem can eat a powerful, long-cast spell or crowd control ability. It's considerably weaker now than it used to be, as it dies even when 'eating' non-damage harmful spells, but always be mindful of its presence. Have a cheap, instant-cast spell at the ready to finish it off so as not to waste more expensive, longer cast spells. Use a /stopcasting macro, if you need to. Make sure the totem isn't around when you blow a cooldown spell like Hammer of Justice.
The Earth Totem Trio
Shamans have three fun earth totems that provide utility in PvP. Earthbind Totems are the most obvious, providing a snare effect (and more, depending on the shaman's spec) but there's also the little understood Stoneclaw Totem, which shamans put down to protect their other totems. Its real value to shamans, however, is because of the Glyph of Stoneclaw Totem, which is the single best PvP glyph for a shaman. Unfortunately, destroying the totem will not remove the damage absorption, so don't bother. It won't be worth risking proccing the 3-second stun. Shamans can put down a Stoneclaw Totem to proc the shield and immediately replace it with an Earthbind Totem. Destroy that, instead. Tremor Totem is an anti-crowd control spell that shamans put down against warlocks and priests, so if you're counting on a fear, charm, or sleep effect, make sure that Tremor Totem is destroyed before casting anything.
Wind Shear replaced Earth Shock as the shaman's interrupting spell and is off the global cooldown, so always watch for it if you're a caster. It has a very short 6 second cooldown, so time your spells accordingly. It creates a 2 second school lockout, so try to bait the shaman into interrupting spells from another school if you have something important you need to cast. This along with Purge makes shamans quite a handful for casters.
Finally, shamans can turn you into a frog. Hex is a 1.5 second cast spell that has a relatively long 45-second cooldown and being the only crowd control spell shamans have, is a great candidate for a cc-break. It can break on damage, so removing it is purely discretionary.
Earth Elemental and Fire Elemental
These totems never see play in Arenas, which is precisely one reason Battlegrounds and world PvP are so much fun! You get to see Earth Elementals and Fire Elementals run amuck, and shamans don't hesitate to blow these long cooldown spells to unleash them upon a crowd. Fire Elementals are the preferred elemental because of their high damage, but Earth Elementals come out when the shaman's Fire Elemental Totem is on cooldown. There's no one solution to these spells because the totems themselves have considerable health and focusing on them leaves you open to attack. Warlocks can cast Banish, though, which should lock up the elemental.
This is old, but it really explains it all. I mean, Frost Shock! While all three shock spells share the same cooldown, Frost Shock is a phenomenal ability because it applies a snare that is not affected by diminishing returns. Theoretically, if you have no means of removing it, you'll be slowed 100% of the time should the shaman elect to use Frost Shock over her other shock spells. We're not even talking about Frozen Power yet. The debuff is magic, and if hampered mobility gets in the way of your play, remove it with abilities like Cleanse but never waste a trinket on it.