Magic is a wonderful aspect of Fantasy. It can add whole new dimensions to the world you are creating by bringing new dilemmas and new problems and challenges. It can also bring a fair amount of spicy and interesting conflicts. But magic is not a panacea. It should not be used to easily solve plot problems and difficulties that your protagonists face. It has to follow rules and you have to establish these rules.

Everything in your fantasy world follows rules. And even if these rules aren't spoken they are implied and understood. Do your characters have to eat? Of course they do! If a character gets wounded in battle is there a price that is paid? Of course there is. Magic must also follow rules. It is a tool and a source of interesting material for your writing but it is not a panacea. And these rules must be understood by your reader.

There is room for a lot of creativity when it comes to putting magic in your writing. After all, it is magic! But you cannot use it as a crutch to easily solve the problems your characters come across. You should use it as a tool for adding richness to your world and for adding another level of problems to be overcome. The way this is done is with the simple rule that magic always brings something with it -something unexpected or unwanted. This something could be unwanted side effects or as of yet unknown implications. Here are some suggestions and guidelines for successfully using magic in your writing.

There is a price to be paid. This is a common technique for managing magic and you see it often. In order for characters to use magic they have to pay a price. This price could be as simple as body weakness and the need for sleep or as complex as the need to drain their own blood to cast spells. (The more potent the spell the more blood that is required.) This puts a limit on the magic.

The unknown consequences to come - Often times a protagonist uses magic without fully understanding the implications and early in a story he or she reaches new heights and later finds out there are dire consequences. This is a useful tool that serves as a way for the character to throttle and monitor his use of magic. This technique works very well because it helps in character growth. Early in the work the character is naive but develops a certain amount of wisdom as time passes.

Corruption of self - The use of magic is often tied very closely with the corrupting effect that power brings. As a character uses more and more magic he becomes morally tainted by it. This is a parable for the adage that absolute power corrupts absolutely.

The loss of something - Sometimes using magic means the loss of something. This could be the loss of physical abilities, loss of humanity or even the loss of the ability to get married. It could be anything and it is a great tool for a writer because it brings up conflicts where your characters have to carefully weigh the benefits and pitfalls of using magic.
So how should you manage magic in your writing?

The first rule of thumb is to lay out the basic rules for the magic early in your writing. That way the reader isn't surprised by the weak writing trick of having your protagonist solve problems by pulling out obscure and powerful spells. Your reader can also enjoy and understand the conflicts as they occur. Second of all you must always be clear on the ramifications of the use of magic. There is always some kind of price to be paid. And finally you must remember that the antagonist probably has just as much right to the magic as the protagonist. There must always be balance. It is a sign of weak writing if the good guys always get powerful spells and the bad guys just have clubs. Keeping this balance of power also makes for much more interesting stories. If you are using magic to solve your character's problems you are cheating your reader. Good magic should bring up just as many problems as it solves.