I'm sitting on the north side of a vast pyramid, deep inside the Temple of Cazic Thule. I came here at the behest of a group of adventurers making claims of the lizard men advancing on their party, and I had grouped with them in the past. Now I'm regaining my magical energies from the last battle, a simple one that only required I slow the lizard that came in. As the paladin gives out a battle cry, my attention is drawn from my meditation and I rise to meet the incoming attackers. Looks to be four, no five of them at once--certain doom for these adventurers, except I'm here.
Quickly I throw a spell of mesmerization over all of the foul things, catching all but one of them. The paladin and rogue break one of them lose right away as they fight, but that's of little importance to me. I'm paying attention to the one charging me now, and a quick burst of color leaves the creature stunned before me as I drop a longer lasting hypnosis over it. Then I return my attention to the last three and recast my shorter duration, group control over them before charming one. I root the other two to the ground and drop my powerful hypnosis over them, and my forced minion attacks the lizard with the tanks, though it dies fast. Wasting no time I root the one that charged me to the ground and send my charmed pet at it, and the tanks figure out quickly which one I want dead next.
My charm breaks free and I quickly stun and recharm the lizard, just as the current creature dies abruptly and the paladin roots the next mob for me--intuitive, I like that. They launch a full assault on the creature and it dies fast, so quickly that I only mesmerize the last lizard one more time during the fight. As charm breaks again I stun it and mesmerize it, then resume my meditations calmly while they finish off the lizard that patiently watched this whole scene. As it falls the healer calls out that her mana is waning, and warn them of the same. The paladin eyes the last lizard, my ally for most of the fight, and though his jaw is strong and his eyes are stoic, I sense fear in his body, though placed in fear for his friends, not himself. "How long can you hold it?" he asks me.
I give a curt glance at the lizard man. There is a bit of drool beginning to leak from the side of its mouth, and I've already regained twice the mana needed to mesmerize it. "Forever, paladin... forever."
There are five primary tools of crowd control; four are the bricks, and the fifth the mortar. Your job is to play the role of the mason, building a fortress of safety from the mobs around you, who will gladly end your life when you make a mistake.
So don't make a mistake.
These are the tools you will hold your enemies at bay with, and the most important role you have in just about any group you join.
So I've told you the tools... now what? It's simple really. You typically use single mez when possible, as it's going to be the most reliable and mana-efficient. If you must get several mobs mezzed fast, you use AoE mez and work on switching when you can. If aggro is a problem, or a mob has been DOTed (or isn't mezzable) Root is there to save the day. When trying to switch out of an AoE Mez situation, or a DOTed mob, charm is a great way to turn the tables on a fight, and in the middle of all this mez, you use PB stuns to keep people healthy while you work.
Learning crowd control is more about interpretation and reaction than planning. The spells memorized are how you intend to control a fight, the spells cast during the battle are purely dependent on the need. Learning to recognize what is needed when is how you become a master. My only real advice to becoming a master of crowd control is to practice. Go to a green conning camp that is no threat to you and try to crowd control it. Open up with an AoE Mez, charm one, and try to kill the camp off without running out of mana. It's good practice because the charmed mob will kill about as quickly as a group would at that level, and you will have charm breaks to deal with. When a mob hits you, you have to back the pet off after it's controlled, giving you an understanding of what the magician has to deal with in group. Use your current spells, not weaker ones that will suffice for the greens. The object isn't to kill the camp, it's to manage your mana while doing it, to get a feel for how many is too many, then push that limit.
Now try dropping a PB Stun amidst those mobs and doing it with 5 things aggro at once. After that, move to a bigger camp. Get a partner with you when it becomes dangerous, and keep practicing. You will reach a time where you look back at advice I've given, and say, "That's why Xornn said to try this!" But most important above all, beyond anything you read here... discover what works for yourself. If you try my tactics repeatedly, and it's not working for you, change it! Maybe your connection speed affects what you can pull off... maybe AoE mezzing just isn't for you... maybe you'll never cast charm in your life, but you love to stunlock! That's fine--just as long as you know you're doing the best you're able.
If you were hoping you'd learn more from reading this section, I hope you can look back and see that I've told you everything you can be told. I'm just hear to teach you how to learn what it means to control a crowd. To actually deal with the pumping heart and anxiety of crunching impacts to your body, you have to be there--you have to do it.