Let me show this by way of example.
Let us, for the sake of example, say that I'm playing a Ley Line Walker. By choosing this Occupation, I already made a decision on what relationship that my character has with the supernatural power that he wields. Specifically, I chose the detached and experimental relationship typical of classic Western High Magic traditions; he sees the study and use of magic as no different than the study and use of the natural world, and comprehends the supernatural as a step beyond the physical world of ordinary existence. He is an educated man, literate and fluent in at least two languages and broadly familiar with an array of sacred and profane knowledge (and I use those terms as real-world scholars do, not in their popular sense; "sacred" means nothing more than "the supernatural world" and "profane" equals "the ordinary world"). He may not be a medical doctor, a physicist, a chemist, an engineer, or some other well-educated professional or highly-trained tradesman in the ordinary sense but he is does possess an array of useful knowledge that can be applied to an array of likely situations. In short, I'm playing someone who is best thought of as a Gandalf sort of figure: an individual who travels far and wide, has many contacts of varying intimacy (acquaintances, allies, friends, relatives, etc.), and often--as a direct consequence of his comprehension and usage of supernatural power--acquires a "Big Picture" perspective on things that can come into conflict with more provincial or shallow-thinking individuals.
A Ley Line Walker, specifically, is a magic-user who acquires a mastery over the useful qualities of Ley Lines. It is this mastery that promotes the "Big Picture" perspective. The ability to nigh-instantly communicate and travel by way of Ley Lines results in the Ley Line Walker acquiring the mind of a frequent traveler, a jet-setter sort, who has no problem being comfortable in places that--in real terms--are vastly distant from one another. Time and distance perspectives become altered accordingly; when your man wake up in Tolkeen, take in lunch on Center, meets up with his on-and-off Atlantean lover for dinner on a planet in another galaxy, and come home to a warm bed in Tolkeen (traversing mind-boggling distances with aplomb several times in a day) it sometimes becomes difficult to remember that the people selling their farm produce in the marketplace regard a day's travel by foot as being very far away. Similarly, when your man can telepathically communicate with his buyer in Atlantis by way of Ley Line communications it is difficult at times to remember that many people can't get fast and reliable communications across a field, let alone across a community. What this means is that I'm playing a character who, for all intents and purposes, is akin to someone who is not only always online, but can teleport ala Star Trek to anywhere on the network.
We haven't even addressed the spell-casting yet.
A Ley Line Walker otherwise uses magic in the manner of a Western High Magician: he studies the supernatural world, conducts experiments to test various hypothesis, records the results, and in time refines things until he develops a technological artifact from that work- a specific spell or ritual. He is as much a valid scientist as any physicist, chemist, or other hard-science practitioner. These findings and applications can be repeated by others, verifying those findings and applications as one would any real-world science or technology (respectively). The only reason for why my character's Stone Golem ritual creates a Stone Golem that does not look like another's Stone Golem is for purely cosmetic reasons. He uses magic; it is a tool to him, nothing more, just as a chemist uses his knowledge of chemical processes and interactions to create smokeless gunpowder. The key difference is in the experience of using that source of power to make his spells work.
Let's take, as an example, casting a simple Fire Bolt spell. I will draw out the specific steps of what your magic-user experiences for illustrative purposes; remember that, for your character, all of this happens in three seconds or less.
- Your character fixes his senses on a specific target.
- Your character fixes in his mind the idea of a small bolt of fire leaping from his hands, flying nigh-instantly to that target, and incinerating him to ash.
- Your character, following the formula that he'd previously mastered, begins to generate the mana required for this spell and transmute it into fire. This requires that your character apply his mastery of vocal techniques, breathing techniques, and guided imagery to accelerate the frequency of the mana that he's cupping in his hand until its structure becomes too agitated to remain in its pure state. The ball-like shape allows an outer shell to remain in place while the internal space transmutes into fire; this is necessary to safely handle the spell effect during the casting of the spell. Your character's vocalization and breathing is how your character manipulates the frequency of the mana, so to others you sound like your voicing the wrath of an angry god or spirit. (They are not exactly wrong; you're mimicking said entity, using principles of similarity to apply that effect.)
- Your character feels the heat gathering about the palms of his hands; this is how he learned to track how long he has to finish his casting.
- Just as the shell holding in the fire is about to collapse from within as the last of its is transmuted into fire, your character "throws" the bolt at the target; this is a somatic ritual component, as the spell doesn't actually generate anything so solid that your character could actually throw it as if it were a snowball or a rock. Your character wills the bolt to strike the target; success or failure is a contest between your will to strike and your target's will to not die.
- Either your character's will prevails, the bolt strikes the target, and he is burned or the target's will prevails and he blocks or dodges the bolt. If the bolt strikes or gets blocked, then the fire burns what it strikes as if it erupted there and then. It has no solidity, so there is no kinetic component; you're just throwing magical Molotov Cocktails. If it misses, it still exists and may strike another thing if within the effect's range.
So, while being familiar with the rules and mechanics regarding what your magic-user can do is important, that is not the end-all/be-all. To fully and properly appreciate what it means to play your magic-user, you must employ your imagination and get into your magic-user's head--into his life--and see through his eyes what it means to be who and what he is (and do what he does). Once you do that, you can better appreciate his perspective, and that means deciding what to do at the table (and in the game) through that perspective. By immersing yourself into your character's reality, you stop being just a guy looking at numbers and mechanics playing a boardless wargame and start truly playing a role; once you become familiar with your man, you'll start wanting to spend time with him more often--and that means playing him more often--which leads to the last bit for this post.
For you, as the Game Master, you're not off the hook. What you need to do does shift a bit, however, in that instead of doing this for a character you will be doing this for entire groups as a whole. Each form of magic-user represents some form of magical society, however formal or informal, and thus has a presence in the environment; this is especially necessary to deal with if you will allow players to play members of that Occupation. This all a part of world-building, so you need to have the vital parts done before the first player rolls up his man. When you know how these magic-users think, act, behave, etc. you'll start seeing emergent creations that you would not have imagined otherwise; this is exciting, so roll with it and see where it takes you- some of the best that tabletop role-playing has to offer stems from such events.