I consider myself a serious arena PvP'er, and these are my favorite tricks for owning it up in the brackets. Most can be used outside the arenas as well.
Mark Targets!
You cannot set up raid targets on players of the opposite alliance, *however* they work just fine on players of your alliance. Very useful for setting up a kill target, as a bouncing skull makes those damned gnome rogues much easier to track. Also excellent for watching for healers that try to go drink - always know which pillar they are behind. I keep a key bound on my keyboard to mark target with a skull - multiple keybindings for different markings can be used as well.
Drink Bug (Credit to Paal)
There is a small delay between casting a heal that puts you in combat and when you actually go into combat, so cast a heal that would put you in combat and spam your drink button to drink right after the cast!
NE - Shadowmeld+Drink (Credit to Sandwhale)
Basic, but if one person didn't know then it was still worth sharing. You can cast shadowmeld after you have started drinking and it will continue drinking stealthed. macro is
/use Star's Tears
/cast Shadowmeld
Other races - keep an eye on those NE mana users!
Sight Debuff to break CC (Credit to Mudkip)
Those purple crystals that allow you to see stealth also give you a small damage tick every three seconds for 21 seconds. That means for 21 seconds, you cannot be sheeped/trapped/sleeped for longer than 3 seconds!
(Is being nerfed 2.4, changed to 15 seconds of taking increased damage instead of a 21 second dot)
Mind Vision Bug (Credit to Moonitoons)
Priests can cast Mind Vision on a target to stop their drinking, regardless of range or LOS. Can you say focus macro?
(This is being extremely buffed in 2.4 with the changes to how drinking works - they will regain little mana for the first few seconds, giving the priest plenty of time to cast mind vision to interrupt!)
2+ Diminishing Returns
Diminishing returns, where a cc will do the full duration, then half duration, then 1 second have a cooldown period of immunity after which they will return to the full duration of cc. This is calculated from the LAST cc cast before the diminishing return is reset. What that means, is if you have 2+ cc, you will gain ALMOST DOUBLE the cc time by casting cc#1 3 times and cc#2 3 times instead of 1, 2, 1, 2, 1, 2.
EXAMPLE:
9 second fear, 6s cyclone, 4.5s fear, 3s cyclone, 1s fear, 1s cyclone
After the target gets immunity to both from diminishing returns, they will have a long time where they cannot be cc'd.
6s cyclone, 3s cyclone, 1s cyclone, then 9s fear + 4.5s fear + 1s fear
By the time cyclone is done, fear will be almost ready again = target spends most of its time cc'd!
In short, cast one cc 3 times, then the other 3 times to get the most out of it.
Some are exploits, some bugs, some are tricks.
I aplologize in advance for any reposts; first posters are credited in titles.
If you have more arena tricks - Share them!
EDIT: added credit to original posters