Mining is by far the most profitable gathering profession in WoW right now and the recent changes that made you only have to mine a node one time to get all the ore from it has made it much faster to level and use. And if you know how, it's really easy to level to Grandmaster quickly.
There are two main methods to powerlevel WoW mining. The first is simply by gathering the metal from the nodes that you find. If you are still leveling your character, simply mine as you quest. This is the least painful method. You will find more than enough nodes to level yourself without even noticing the effort. If you are already level 80 and want to powerlevel Mining, you should find zones that have a lot of they highest type of node that you can mine and plan a circuit around it.
The best way to do this is to go to Wowhead or other WoW database site and search for 'metalname vein' or 'metalname deposit' i.e. 'copper vein' (or you can search for the metal name then click on the Object tab in Wowhead). Click on the zone that has the highest number of nodes and look at the map to see where nodes spawn. This map will allow you to plan a circuit that takes you to all the main areas where you can mine. As a general rule, nodes spawn around zone edges and on hills/mountains or rocky outcrops. Database sites will also tell you which nodes you can mine at which level. As soon as you can mine a new metal type, move on. Make sure that you don't forget to train before you hit the cap. Remember that smelting the metal will also get you skill ups, but usually only at the lower end of the skill range for mining a metal. For example, you can only gain 5 skill ups from smelting Tin before it turns green.
Another method that you can use is to take advantage of skill ups you get by smelting. This method require you to have some gold to throw around already. If you don't have a lot to spend, you can use this method to get through low level mining faster and then switch to the gather method outlined above. Purchase ore from the auction house and smelt it until the recipe turns grey. There will be gaps where you will need to gather and you will need to smelt the rare metals like silver, gold etc but this method can save you a lot of time.