I am currently developing the mechanics for magic in my story-world so that it (and the magicians who use it) acts as a believable system which is governed by reasonable rules.
The easiest way for me to do this is to apply the laws of physics. So... I take the basic premise that the world/universe is made up of two forms:- Substance and Force, and further break substance into Matter and Energy.
I initially decided to have magic as being simply a form of energy which complies with the energy conservation law (energy cannot be added or subtracted, only transformed). Physics tells us that during energy transfer, Work is done. The products of Work are Heat, Light, Sound and Motion. So I can see that Magic-as-Energy is a believable system, I'm just adding a new form of energy to the pool.
Keeping magic as simply energy - or only energy - opened up the question of how a person can purposefully manipulate that energy to generate the required form of Work.
Force is applied to Substance to alter its shape or the way in which it is moving. It can be represented as pressure, acceleration, turning and momentum. So what if a person has the innate aptitude to apply a force to magical energy in order to produce the desired work?
This, for me, has opened up a fascinating and almost limitless world of possible ways in which magic can be represented, manipulated and harnessed. It cannot become a Deus ex Machina used to simply override plot problems because the magician can only use it according to the laws of physics - the most important one being the Law of Conservation of Energy. I have an amazing range of detail and potential for this concept, ideas which I don't want to put on the 'net because I don't want them stolen. I'm actually a little worried about putting this much up. But then not that many people visit my 'blog so I think I'm reasonably safe.
I'd like to know what you think of the concept, and how you would apply it. But be careful - I am a hypocrite and I am likely to steal your ideas for my own use ;-)