DISCLAIMER: I have known about this for over a year, and it seems so obvious that I'm sure it has been posted before. However, I searched and I did not find any related topics (I'm guessing it's in the Elite section, but since I cannot view those posts I have no way of knowing). I am not claiming to be the first to have discovered this, but I did discover and test it on my own, so please no flaming.
Most people know that in raids you can get three Healthstones by having three Warlocks each with different number of points in Improved Healthstone. However you can do this same trick on a single Warlock, by conjuring a Healthstone, spending a point, conjuring a second Healthstone, spend last point in Improved Healthstone and conjure a third one. Although healthstones are Unique, they are only Unique for stones with the same level of Improved Healthstone. They still share a cooldown, so if you pop a Healthstone, you won't be able to use the next one until the cooldown is up again. (Also, if you pop your Healthstone early you can run back and grab another one from the well, if yours in 3s or 5s that is).
The downside is that it requires you to respec, and can only be used for one game. However, most Warlocks respec for Arenas every week as it is, so this could be practical for at least the first Arena each week. The other concern is that it can take several seconds to spend all your talent points. (Anyone know any macros or addons that help you spend talent points?)
This was tested on 2.4.2 in Arenas on a live server (can someone confirm for 2.4.3?). Sorry, I don't have any screenshots to post at the moment, but the instructions should be easy enough to follow. Although I am posting this as an Arena exploit, it can be used anywhere. After you respec back to PvE on Tuesday, you can conjure 3 healthstones for your next party or raid as well.