WoWWiki has a great guide to raising your reputation from scratch. If you head down to the Spawning Glen, Quagg Ridge, Funggor Cavern or the Dead Mire, you'll find mobs that you can kill for up to 15 rep a pop. Even at level 70, provided you are farming mobs level 62+, you will be able to gain 15 reputation from kills. Again, possessing the herbalism skill allows you extra loot when farming these mobs. Even without this, you can acquire motes of life as well as unidentified plant parts to hand in for Cenarion reputation, send to an alt, or sell on the auction house.
You can also find mobs in Slave Pens that will yield rep when slain. While this method is great for bonus reputation if you're after these drops, the numerous quests, especially the repeatable ones, will yield more reputation per kill, on average.
If you are beginning your grind at unfriendly, many of these mobs will also drop Bog Lord tendrils, which can get you to friendly in a mere 8 turnins. You can also pick up mature spore sacs in the glen. Both the sacs and the tendrils can be handed to Fahssn, who wanders the eastern edge of the glen. Once friendly, you can then begin to talk to the Sporelings, and these items can no longer be turned in. There is also a quest that can be obtained in the Cenarion Refuge that involves killing mobs in Funggor Cavern and the Dead Mire, which will yield Sporeggar reputation per kill.
A neutral quest is also offered in Sporeggar, requesting glowcap, the Sporeling currency. Although you might be tempted to save these, they grow abundantly throughout Zangarmarsh, so turn them in while you can.
If you visit Gshaff, then you can start handing in sets of 10 fertile spores for 750 rep, 10750 experience (or about 5g at level 70), and a glowcap. This quest is repeatable for a limited time, so take advantage of it while you can use it, and save your sanguine hibiscus, and dungeon quests, for further into your reputation grind. The fertile spores drop off of spore bats and marsh walkers, and can be found in the auction house.
Be sure to visit the Cenarion Watchpost (near the Spawning Glen), as well as Sporeggar, for quests that you can pick up. These often involve killing mobs that yield extra reputation when killed, and drop items for repeatable quests, as well as giving you reputation for the primary quest.
Try to get as far into your reputation grind as possible using fertile spores, Bog Lord tendrils, and early or non-repeatable quests as possible before you delve into the main grind. In addition to collecting sanguine hibiscus, you can also farm naga for the repeatable quest "Now that we're still friends...". You must kill 12 Bloodscale Enchantresses and 12 Bloodscale Slavedrivers, and alone would require over 1000 kills to hit exalted. Fortunately, you can break this up with Underbog runs for sanguine hibiscus, as well as take a dent out of your total by completing the various quests described above.
Follow this guide, and you'll be flapping with your Tiny Spore Bat, sporting a new tabard, in no time!