Use your Alts Wisely

Serious gold making players almost always have a line-up of alts to leverage the different professions available to them and it's important to make the right choices for which professions to take. It's not critical to have every profession, but the difference in profitability increases exponentially the more markets you have available to you. So what's the best way to go about choosing which professions to take?
Do your market research for your particular realm. Not all realm economies are created equal, and for that matter the faction economies on each realm tend to vary widely. Use Auctioneer scans for a few weeks to get a good feel for what will sell and identify areas that are currently under served by your realm's crafters. Professions that pair well together:

Alchemy/Herbalism
Skinning/Leatherworking
Jewelcrafting/Enchanting
Mining/Jewelcrafting
Mining/Blacksmithing
Enchanting/Tailoring
Herbalism/Inscription

You don't necessarily need to pair professions like this, it all depends on your server. If cheap mats are consistently available on your AH, then replacing a gathering profession with another crafting profession may be the way to go. However, one thing that should be avoided is redundant professions. I read a post earlier this week from an alleged gold guru who stated this was his gold making "machine":

Toon1 - Mining/Jewelcrafting
Toon2 - Mining/Blacksmithing
Toon3 - Mining/Alchemy
Toon4- Mining/Tailoring
Toon5 - Herbalism/Inscription

To me, this seems like a lost opportunity. Why have 4 toons with 450 mining when one can do the job equally well? I would drop mining on three of them and replace them with Transmute specced alchemists to take advantage of the epic gem transmutes, leaving my list looking like this:

Toon1 - Alchemy(transmute)/Jewelcrafting
Toon2 - Alchemy(transmute)/Blacksmithing
Toon3 - Mining/Alchemy(Elixir)
Toon4- Alchemy(transmute)/Tailoring
Toon5 - Herbalism/Inscription

Even with the recent changes to the epic gem market, this would still prove a more profitable set-up.