First I'd like to say this: Don't underestimate the power in destroying the teamplay of your opponents. Usually this can be achieved with a slowly increasing pressure by gradually advancing towards the enemy team. This increasing pressure induces stress and will in many situations lower their ability to get all team members to stick to the plan and play at their best.

In TBC you could wait out your opponents, let them come to you.. but I find that in WOTLK it doesn't work very well anymore. When you do this, you are making sure that they have a very good plan and strategy, and you give them time and room to execute it properly. The reason this doesn't work in WOTLK is due to the vast amounts of AOE CC that is available to teams to use against multiboxers. Used properly, your team can be totally shut down for 10-15 seconds. It is VERY important to avoid this situation, and deny them this opportunity, in my opinion.

I find that I'm losing against most team that I "wait out".


And shaman have been given some awesome tools to take control of fights: Thunderstorm and Fire Nova stun.
This can be heavily abused in several arenas: Lordaeron (at the grave), BEM (on bridgeheads), Dalaran (at the edges).



So my general tip would be: Make sure you pressure teams into making mistakes, destroy their coordination. Don't let them set up AOE CC rotations. Make them panic and waste their cooldowns.. LOS objects are your friends now, because you can get people out of them by rotating TS.
The key thing to note is that, when you are chasing people that are hiding in LOS, they are spending time running rather than killing your shammies. Push them out of LOS and kill with LVB + Shock.
Also, pre-charge Elemental Oath buff with heals or flame shocks. This is especially good on Orgrimmar arena.