When Wrath of the Lich King hits the new profession people will be urgently trying to level is "Inscription". Inscription requires a ton of herbs, including many low level ones and pre-bc ones that no one ever farms or uses anymore.
Do you remember the price of low level gems/ore when BC hit for the first few weeks? It was insane, and Wrath will be no different. Like "Prospecting" for JC, inscription has "Milling", using 5 herbs and making a type of pomace, used for making inks that do different things. This isn't as much of a guide as it is pointing out the obvious that MANY people will overlook. Start buying low level herbs now or farming them, because once wrath hits people will start emptying their pockets in attempts to powerlevel the new profession and cash in on it while they can. This is when you cash in on their oversight.
To sum it up:
Step 1: Buy a ton of pre-bc and low level herbs that are cheap, or go farm them and save them up on a bank alt. Many high level herbs will be needed too, but some of those still go for a hefty sum at the moment.
Step 2: Wait for Wrath to hit.
Step 3: Profit.
Hopefully not too many people who read this will be on my server. I'd hate for low level herbs to start skyrocketing in the next few weeks.
Also, I checked for posts about this and found none. If it's a repost, my apologies, but I didn't find anything.
Enjoy.