That I would tell you when I hit the button? Well that button is going to be pushed today. This means the following:
Get rid of things you wont have a use for and don't have the space for. So you have a tank set and you never tank. Get rid of it. You have a healing set and never heal, same thing. Clear out your bank of things you can part with. If you cant then this is the time to make sure you have the space required to keep things. Send non BoP things to alts for holding. Try to sell epics that still have a value. If they don't then bit the bullet on them.
The key to leveling fast and getting ahead of the curve is to be prepared. I will be making a god awful amount of bandages for leveling over the coming weeks, because bandages for warriors make down time during leveling so much less. Flasks make sure you have some. They will be really cheap and will make you kill things faster therefore increasing your levels/per hour. Buff food same thing as flasks, and of course food in general to make down time much less. Really anything to make your down time less is a benefit so collect all you can i nthe time you can.
What is the easiest way to get ahead on the leveling? Complete your quest log to the max with quests with an easy turn in. This means 25 quests and 25 dailies. For people like me there probably isn't 25 quests left so try to get as many as you can. Yes they wont be as much XP as the new quests, but XP is still XP so no reason not to.
Professions leveling at the start of an expansion is extremely extensive. This is of course referring to crafting professions. Learn to take advantage of this on all levels. With 2 new races take the opportunity to make gold off of people starting from the get go. So low leveling mats will skyrocket in price with the amount of rerolls. You would be crazy not to take advantage of this. So be conservative with your own when the expansion hits, but now take the time to collect up those mats while they are cheap on the auction house to make a big buck later on. Blue gems, shards, dusts, cloth, and meat will be the big ones.
Realize this. The amount of gold you have today is not going to be worth the same amount come Dec 7th. So realize maintaining what you have is actually making yourself worth less. Inflation will be a killer and there is talk about this expansion's inflation being the worst of them all.
So good luck. I hope this helps in getting you prepared.