Herbalism is the art of picking flowers. It is also the art of making cash, which is what we want :)

Herbalism is one of the few professions that will net you money with little effort. All other professions require you spending a lot of materials to level them up, thus you need to do a lot of business to even break even.

It's highly recommended that you wait until you have a mount before you start leveling herbalism seriously. Up until then just pick whatever flowers you stumble upon. In fact, forget about professions (except skinning) until you hit 55+, since you can't pick the profitable flowers until then.

The best way to level this profession is by being patient. Get a 5-10 minute route down, ride it, then log for an hour and repeat it.

1-15: Here you will collect Silverleaf and Peacebloom in the newbie zones. Easy.

15-50: Earthroot. Alliance head to Dun Morogh, Horde to Durotar. Maps: http://thottbot.com/?s=earthroot

50-85: Mageroyal. Alliance head to Loch Modan/Westfall, Horde head to Barrens/Silverpine. Maps: http://thottbot.com/?i=658

85-150: Stranglekelp. Here you have two options: Wetlands or STV. Personally I prefer Wetlands. For Wetlands swim up and down the coast a bit out. For STV either swim up the west coast from BB to the Zandalar tribe isle or swim up and down the SE coast. Maps: STV: http://thottbot.com/?m=8889 and Wetlands: http://thottbot.com/?m=9239

150-195: Liferoot. Wetlands is the best option for this part. Ride around the western swamp area; from the coast to the road leading to Dun Modr. Interspersed with the liferoot is also Kingsblood, which sells decently on AH due to it being needed for Thorium rep. Maps: http://thottbot.com/?m=9232

195-235: Wintersbite. The only place for this herb is in Alterac Mountains. When coming from the south (either TM or SS), first ride through the yeti area of the mountains, then ride through the ogre stronghold and finally ride through the ogre part of the mountains. Log and wait 30-60 mins, then repeat in reverse order. This part is pretty quickly done. Maps: http://thottbot.com/?m=19247

235-270: Blindweed. I prefer using Swamp of Sorrows for this part, since you can get a nice pattern going easily. Simply circle the central lake, log 30 mins and repeat. Alternatively ride back and forth through the northern part of the swamp. Maps: http://thottbot.com/?m=21505

270-300: Dreamfoil. This is the reason you level herbalism. Dreamfoil is used extensively in all the top alchemy recipes and sells for a pretty penny. .75-1.5G per flower depending on server. There are three main places for this herb: Azshara, Felwood or Un'goro. Try all three places and find the least camped one. Dreamfoil grows in open spaces, so stay away from rocky areas. However, Mountain Silversage (used in health pots), grows in mountains, so find a few spawn points of this herb and ride past them on your rounds.
Of the three Un'goro seems to be the least camped...but again it varies with server. Personally I use Azshara to pick my Dreamfoil and it works quite well.

Maps:
Azshara: http://thottbot.com/?m=70549
Felwood: http://thottbot.com/?m=47575
Un'goro: http://thottbot.com/?m=36626

Other herbs worth picking:

Icecap: Used in flasks and mana pots. Only grows in Winterspring in open areas. Heavily camped. Maps: http://thottbot.com/?m=28935

Mountain Silversage: Used in Mongoose pots and health pots. Grows in mountaneous regions. Un'goro is the spot for this herb. Alternatively Winterspring if you are after Icecap.Maps: Un'Goro: http://thottbot.com/?m=33245 and WS: http://thottbot.com/?m=28933

Plaguebloom: Used in Mongoose pots and various elixirs. Grows in both plaguelands and Felwood. Maps: EPL: http://thottbot.com/?m=14671 and WPL: http://thottbot.com/?m=26432 and Felwood: http://thottbot.com/?m=1011788

Gromsblood: Used in Flask of the Titans. Grows in Desolace, Felwood and Blasted Lands. Maps: http://thottbot.com/?i=5537

And there you have it. Herbalism is very good money with very little actual work. You only have to ride around :) When you decide to sell your gatherings, check your AH first and undercut a bit. If the bottom drops out of the market, jsut wait a few days and stockpile in the meanwhile, then sell when prices move back up.