When a new expansion is introduced, you will always see people switching their main characters. Either they got tired of their old class, they want to try out a new role, or they believe that that shiny new alt they rolled is going to be a better fit for them. With Mists, you can also add the reason: “help, my class has been completely overhauled”. You will find that there’s a lot of catching up to do, and you don’t really want to wait until the next raiding season to practice all those cooldowns, rotations and gimmicks.
So here’s what you can do: find equivalent practice.
In the basics, it’s looking for a training experience equivalent to whatever you are practicing for, except easily repeatable and sometimes secluded for better number crunching. It’s like stepping into a simulator to train your muscle memory for the operations needed to drive a car before setting foot into a real vehicle. You already get the kinks out before stuff gets real.
The first step in learning your rotations is to map them out: find a good blog, elitist jerks or a skilled friend and find out what buttons you push and in what order. A simple way of doing this is to make a sort of flow chart on a piece of paper, where you put the abilities you cast into boxes and connect them based on 1) what needs to be up in what order of priority and 2) what needs casting based on a prerequisite (such as SW:Deathx2 below 25% health).
You then map out your button bar so that it reflects this chart; in other words, the most important and often used abilities go at the start of the bar, in easy reach. I like to keep Fade and Shield at the back side of the bar, still within quick click range, in case things go wrong. This is a good place for DPS cooldowns as well, such as Archangel and Shadowfiend. Then comes the next step: practicing at a training dummy.
Training Dummies, while awful for finding out what your DPS would be in a raid or the like, are really amiable targets for repeated and continuous spamming. So basically grind out your basic rotations for half an hour or so, trying to get the best grip on it you can. Because there are no mechanics to worry about, you can easily focus on your buttons and bar to work on a good sequence and timing.
Many people would now consider the next step to go into a dungeon – however for reasons I will mention this is usually a bad place to practice these things. Instead, join up for an LFR run. Since with moderate skill you can already do really well there, and mechanics (while present) are not really dangerous, you can afford to spend more time working on your casting sequences. I imagine that this will be no different in Mists of Pandaria, although it does mean you will need to already have reasonable gear for your toon.
Training burst damage and spatial awareness
For these things, the random dungeon finder is awesome (or even better: queueing up with friends). Assuming you know the basic rotations well enough now, this is the place to learn how to apply burst damage and train spatial awareness. Since people in dungeons are often overgearing the place once the raid seasons started, you will have less chance to practice sustained DPS. The more damage the other people are doing in your group, the less time the mob has to live – and the less time you have to practice.
There’s also the matter that low DPS and attempting to learn a rotation while in a random dungeon (especially heroic) is going to make people really edgy. Most people there just want to rush through for gear and Valor points, and they aren’t really waiting for someone who’s still learning the ropes. However, the fact that the heroics/dungeons have proper encounter mechanics to follow means that you will get practice in spatial awareness and performing your role while paying attention to your surroundings.
Survival and creativity training
To be fair, nothing beats learning to survive and creative use of your skills as does PvP. Random battlegrounds are a good start, and you can easily jump right in with a starter kit made by a crafting profession. In the beginning you will die a lot, but as you become more savvy and get better gear you will live longer and be a more threatening target. This is great to learn what possible means of survival you have when playing that class.
Second, since you fight against Human opponents, you need better and more surprising tactics to fool them. This means you will learn by instinct what abilities (and combinations) will work to lock an opponent down or leave them behind while you escape. All skills which will be valuable while raiding.
Class and spec-specific tricks
These are perhaps the only things that can only be learned by using them in arenas, rated battlegrounds or raids. These are either class-specific tricks (Prot warrior charge/intercept, hunter kiting/jump shot, Priestly Hymn of Hope + Shadowfiend) or role-specific (corner pulls, heal stacking, multidotting) and you are going to have to learn them the hard way, by experience. Often it’s these tricks that make the difference between a good DPS and an amazing DPS, and you will learn a lot instinctively and with experience.
There are plenty of ways to train yourself in your class before the raids start – and with Mists of Pandaria bringing in a breath of fresh air for a lot of classes, even experienced players might benefit from a few simple tips on how to (re)learn critical skills for raiding and PvP. If you identify yours now, you can be up and running while your fellows – I mean competition – are still learning to walk again.