in my series/world I have one book with magic users in it. It's the only time magic shows up in my world, but it's the main element of the book/story. I portray magic as 'evil/bad' in my story. Partially why it only shows up in one book.
I finally got around to thinking about how/what kind of magic they would use. This is the idea I came up with (I actually think I got it from a dream I had, but that's beside the point........)
Magic (it may have a different name in my world, still working on that) was 'discovered' by a man/Ashtain named Teynon. At least he thought he discovered it, magic had really been 'revealed' to him by Natorii (one of the fallen E'shawnti) in another attempt to wipe the Ashtains (my main race) from the world.
*history that's not really relevant*
They use magic by drawing a 'spell' in the air and collecting, aiming, and sending it at the enemy. They draw with their pointer and ring fingers crossing over each other. The ring finger is the actual 'tip' they draw with. 
There many ways to draw a spell and get the same result. Newer magicians have to spell out the spell more clearly to make it do what they want, where as older/more experienced magicians can write the same spell with much fewer words/symbols.
When a magician draws a spell it remains in the air. (like an echo from staring at lights at night) That way they can see what they've already written.
As far as collecting, aiming and sending goes; think of Ironman(2) the movie when he's in his lab in the basement and the computer is projecting all the pretty images. Collecting a spell is like grabbing the projected images and crushing them into a ball. This is how the spell is 'mixed' or 'made'. Aiming and sending are almost the same thing, after the magician collects the spell he chooses the enemy to attack and 'throws' the spell at him.
As far as what they can do with magic, anything from physically wounding someone to creating explosions to distracting a person with a image of someone they love.
Right now the only factor I have limiting their magic is how fast they can write and throw their magic. Any ideas of how I can limit them otherwise? I don't really want to have it depend on their energy (it seems everyone is doing that 

 ) or any comments on how I refine their magic?