This is the best Herbalism Guide on the entire internets. Herbalism powerleveling will actually make your gold, instead of cost you plenty of gold like power leveling crafting professions. Below are some steps followed by instructions explaining where to go for each level of herbalism.
If you're stuck leveling your herbalism (which you won't be with the below maps). 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 while, we'd recommend you get an epic ground mount first. You'll find trainers in every major city to learn herbalism. At last, make sure that you have your 'Find Herbs' tool activated by left clicking on the hour glass connected to your minimap: