Farming Essences

Farming essences is a great way to make gold.  These can be great, especially if you make smart decisions about when to sell them.  If you aren't in a rush for cash, you can keep them in your bank until prices are at a high, and then bring them out and make a killing.  So there are 3 different spots that are good for farming essences:

1. Toxic Horrors in Felwood - You can look up the mob name on your favorite site to get the exact location if you don't know where they are.  There's a number of these mobs and they drop Essence of Water on a pretty regular basis.  They do poison, so you'll want to bring some potions or food or bandages (or all of the above).  Of course, with the poison it'll interrupt bandages.  Generally they die pretty quickly so it's not too much of an issue.  Also, there tends to be a lot of competition for these mobs, so if you are set up to tag the mob at range you'll be better off.
2. Fire Elementals in Un'Goro - these are my personal favorites because you don't have to mess with swimming through all the water like you do in Felwood.  The elementals are in the mountain in the middle of the zone.  There are a lot more mobs than in Felwood as well, and they have a decent drop rate of Essence of Fire.  They also drop other items like Heart of Fire or Elemental Fire, and I get random greens as well pretty often.  If you're lucky you can get blue drops, or even the very occasional purple.  One other tip here - don't bother with the big guy at the top of the mountain: it's a lot of trouble without the reward.
3. Dust Stormers in Silithus - these are a little tougher than the mobs listed above, and there are also other mobs around for you to worry about.  The payoff is that Essence of Air, which these guys drop, usually sell for more than any of the other essences.  When you're fighting them they will occasionally cast a lightning cloud over your head.  Make sure you pull them out of it right away so it doesn't keep hitting you.  The other advantage of these is that there's so much other stuff going on down in Silithus, that you aren't really way out of your way like you are with Un'Goro or Felwood.  If it's really, really camped you can always find other things of reasonable level to do. 

 It's a good idea to find a rotation that works for you, that gets you back to the first mob you killed right as it spawns.  This is not really important if you are the only one there, but especially with the first two there's a reasonable chance that other people will be there farming as well.  So try to get going in a loop.  If you're doing it right, the other people won't really have a chance at killing 'your' mobs, because right as they spawn you show up.  If you have the timing down, you'll get the lion's share of the kills and make a lot of gold doing it.