This guide is Horde only and is geared to the levels 36-40. Once you are run out of WOW gold to buy your mount when you get to level 36, and for my experience I figured out various tactics to make World of Warcraft money. Hope it's useful to you.
Rock Elementals
Go to the Badlands and went to the camp at the northwest and the east there are level 37-40 rock elementals. This is pure grinding, kill them and keep the items they drop. Vendor everything but the 'Solid Stone', 'Deep rock Salt' and the 'Elemental Earth' because at the AH you can make a much larger profit of WOW money. This is a real good place to grind.
Scarlet Monastery
You have to grind together with your friend. Get a level 60 friend to run the armory and library over and over (Cathedral if you get 2 high levels) and keep the loot and cash. A run with friends that give you all loot and WOW money they earned at end ranges from 7-11 WOW gold. Another way is to just run it with a normal group, the loot isn't as good but each full run of every part of the instance gets you at least 3 WOW gold (And a whole lot of experience!). The ultimate grind place for cash is the graveyard in Scarlet Monastery. If you can get yourself past the elite bad guys in the beginning, and into the graveyard proper, the Forsaken Spirits drop from 1 to 8 silver, along with greys and the occasional greens, and they're Levels 30-32 non-elites. They come in packs of three, and a rogue has little to no problems taking them down hard and fast.
Stranglethorn Vale
Stranglethorn is gank city, but it isn't as bad as people tell. Just head to the north east and farm the Adventure Co. Geologists. They drop a lot of silk that can be sent to the AH and drop quite a few greens this is great for WOW money, but it gets old fast, doing this for too long can be very boring. Another good spot is the Murlocs, they always have chests all around and are easy to kill. They have low def and don't hit hard so you can take a good 3-4 of them down without resting! These guys always drop 1-3 silver as World of Warcraft money which is pretty good if you ask me.