As we all know this is the best Herbalism Guide on the internets. Power leveling herbalism will actually make you money, instead of cost you tons of gold like power leveling crafting professions. Below are some tips followed by instructions explaining where to go for each level of herbalism.
If you're stuck leveling your herbalism. Consider the level of your herbalism, say 240. Divide it by 5, so level 48. Subtract about 5 levels and head to a zone of that level (Tanaris/Feralas) and you should have decent luck leveling your herbalism there. But herb densities won't always be optimal.
If you're going to power level herbalism in a short period of time, I'd recommend you get an epic ground mount first. You'll find trainers in every major city to learn herbalism. 
Levels 1-75
Once you're ready to go, head to any of the starting zones like Durator picking up Silverleaf, Peacebloom, Earthroot and Mageroyal until your herbalism skill is 75. It took me less than an hour in Durator to get to 75.
Levels 75-150
Once you reach 70 or 75, you can head to Darnassus and train Herbalism so you can exceed level 75. Then head to northern Barrens and run the following route until you're level 100, then head to the southern Barrens. Southern Barrens will get you to level 150. Levels 135-150 will be a bit slow, feel free to run around Stonetalon Mountain or eastern Hillsbrad Foothills if you want new scenery.
Levels 150-170
Head to the wetlands and level to 170 by collecting Briarthorn, Kingsblood and Liferoot. Running the route from the Harbor to Arathi Highlands with zig-zags should get you the 20 levels you need
Levels 170-225
Arathi Highlands is an awesome place to level herbalism once you get to 170. You'll spend a lot of time there gathering Wild Steelbloom, Bruiseweed, Fadeleaf, Goldthorn, Liferoot and Khadgar's Whiskers. Stay there until you are level 225, and don't leave earlier because you'll have a rough time finding a more efficient place to level.
Levels 225-275
Head to Feralas and start gathering Goldthorn and Liferoot until you're 230, then you'll be able to start collecting all the Sungrass you keep running by. Collect Goldthorn, Liferoot, Sungrass, Purple Lotus, and Golden Samsan until you are 275. You can see by this map where most of the herbs were for me when I ran through.
Levels 275-300
Finally, we'll head to the plaguelands to level to 300 before heading to the Outlands. Start in Western and then head east. You will run by a lot of plaguebloom, just mark it with gatherer by right clicking it so it says 'You must be level 285 to..' and keep going. Once you hit 285 turn around and run by all the plaguebloom you saw and you should hit 300 before returning to Chillwind.
To some extent, this guide is actually a bit dated, seeing most of the herb level requirements changed and Fadeleaf doesn't sell well anymore. However, the guide may help some players get their desired level quickly. Enjoy this guide, enjoy yourself.