The advantage of WoW grinding spots over questing is that you don't lose time traveling from and to the quest givers, or searching your quest target. Many people try to grind monsters of their level, or even a level or two above them, because these monsters give the most exp per kill, But this will do that, not only do the fights take longer, you are also likely to end the combat with less health and/or manna than you started with, necessitating some downtime between fights. So the most important grinding advice is trying to find mobs a bit lower than your level, which still give good exp, but which you can kill a lot faster and with less downtime.That takes a bit longer for the kill, but during the wand phase he regenerates manna, and ends combat with full manna, directly ready for the next pull again. There are a couple of add-on's, like Telo's Infobar, which are able to tell you how many exp per hour you gained in your current gaming session. These are valuable tools in getting grinding right. Finding the optimum between killing speed and exp per kill isn't always easy; you'll better experiment a bit. You might need to quickly log out and log back on to reset the counter if you want to test exp per hour of a particular batch of monsters.

If you have a warrior, experiment with the different stances, sometimes using defensive stance is a good option, because again you end the combat with more health, and have less downtime before the next combat. If you can, grind mobs which give reputation points, for example the furbolgs which give Timbermaw faction. Unfortunately there aren't all that many faction giving mobs, and they are usually heavily camped. A good place to look for mobs to grind is the various caves, for example with yeti or ogres, because the mobs there are often quite close to each other, so you don't have to walk far between two combats. If you happen to be a miner, these caves often also have mineral ores in them, for an added bonus. If you happen to know a quest that requires killing exactly the mobs you wanted to grind anyway, take the quest as bonus reward.