So I've been contemplating this blog post for a long time. Truth be told, the idea is a few years in the making. But let's see if I can't make some progress on it. This blog post will be quite incomplete, but hopefully it'll give you, faithful readers, and me a sense on what's out there.
Magic is a tricky thing. It's a staple of most fantasy, and yet every author puts their own spin on it. This makes it tricky to write, since there's so much possibility. The purpose of this entry is to try and break it down into categories, with examples, and maybe some commentary. So let's get started.
Magical Effects
The focus of this entry is on active magic, but I would be remiss if I didn't discuss, at least at some level, side-line magical effects. This is probably some of the most common magic out there, because it requires the least amount of though on developing a coherent system with it's own rules and drawback.
In this sort of magic, the effects are usually environmental, rather than intentional. Wayside school has a missing 13th floor, and nobody gives it a second thought. The Indian in the Cupboard is another example, where all of the magic is contained in the little cupboard. For the ease of discussion, I'll lump many werewolf and shifter mythologies in here as well, since often times the main drawback is lack of control. In scope, the magic is often either very limited, or very broadly applied. In both cases, it offers little flexibility.
In writing these, the effects are only as important as the conflict it draws for your characters. If bringing pumpkins to life makes your characters life harder, and if it fits the theme and mood of your story? Then consider adding it, because everyone loves talking pumpkins.
Focusing Will: Word
So now onto our exploration of more active magics. In most cases, magic is an exercise in will. The system, be it incantations, runes, whatever, simply serves to focus the will. In some cases, the Dresden Files being the most recent example that's crossed my bookshelves, you can do magic without the words, but it's usually harder and always more dangerous.
One of the most common ways of focusing magic is through incantations. In most cases, the language used to focus the will is important. It can be an old language (Eragon is one example, though it also uses aspects presented later. Another is Name of the Wind, and a third is Earthsea, and who can forget Harry Potter). Many times, an ancient language appropriate to the setting is used, though it does occasionally happen that a language specific to magic is created. Another possibility is that it can be any language except your native one. I know I've seen this last one in a few cases (Dresden touches on it), but I can't think of a clear example. In most cases where this last one is used, the explanation is that your native language is too closely colored by your own interpretations to correctly formulate spells. A third important category is where your native language is used. The main example of this I have at the moment is The Belgariad.
Blood
Another common motif is the importance of blood. In some cases (The Hollows books), it's enzymes in the blood that kindles magic. In others (one of the systems in the Dragon Nimbus Histories), spilling blood strengthens a spell, offers more power. Sometimes (Meredtih Gentry books), the blood is simply the target of the magic itself.
The thing with blood, is that it holds life. In any magical undertaking, its a powerful symbol. I don't think I've seen a single one where blood was present and unimportant.
True Names
True names are another big power. Giving someone your true name gives them power over you. In some cases, this is why practitioners take crazy monikers. In some cases (The Hollows, Dresden Files), true names are only a problem for sentient beings. In some cases, this is restricted to humans, in others it can include supernatural creatures as well. In other cases (Eragon, Name of the Wind, Earthsea), everything has a true name, and it is by speaking that name that you gain power over it. In still other cases, magic is worked by changing the true name (Earthsea has some examples of this).
Sympathy
Another interesting method is using Sympathy. Name of hte Wind is the one that comes to mind that uses this to the greatest effect, but it's present in the Dresden files, and any other piece that uses the vodoun/voodoo tradition. The idea is that magic is transferred through similarity. Pieces (hair, skin, blood, nails for people or animals, a branch for a tree, etc.) are the best similarity for a whole, but sometimes other things can be used. The more similar the items, the less effort it takes to create an effect.
Math
Sometimes, magic is ordered by the structure of math. I'll also toss programming languages into this category, because what's important is that it's structured. In this case, magic is really a exercise in adding order to the chaos of reality. Some examples of this include the Mathemagics, Wizardry Compiled, and the WebMage novels.
Runes
The last method of focusing will I'll talk about is Runes. These are shapes, often very specific ones, which are used to focus energies into magic. Name of the Wind has some sigildry in it,
School for Adventurers and the Mortal Instruments also use runes to great effect. Many modern occult practices also set great store in symbols and runes, rooted in the ancient nordic and celtic traditions. It's also worth noting that a special case symbol, the circle, is often used as a barrier for protection.
Sources of Magic: Internal
The next topic on the agenda is where does hte magic come from. Probably the most limiting, and yet still very common, methods is to have Magic be internal. It is usually a finite quantity inside the willworker. When the magic is consumed, the mage usually is weak, almost on the brink of death. She has spent all her will, all her life in trying to achieve her aim.
Elemental
Another common source is the elements. Usually, its the standard 4: Earth, Fire, Wind and Water. occaionsally another one will pop up (Heart, Void, Spirit). Sometimes it can be even more specific, including most anything that can occur (Stone, Plants, etc.). The last one bleeds into the next category. Some examples of Elemental-based magic are Warrior and Witch, the Darksword books, and Weather Warden.
Environmental
Sometimes the magic can be drawn from anywhere in the environment, or from specific things. Some of the Magic the Gathering books involve pulling energy from the land. This was also present in Dragon Ball Z a few times. Some cases are more limiting, such as the Circle of Magic books. In those cases, specific objects, ideas, or events can grant magical power, but if you're locked out of those, then you can be in big trouble. The Dragon Nimbus books are another extreme example of this, where the magic can be drawn from dragons.
Ley Lines
Sometimes the magic comes from very specific areas of the earth. While there's no common explanation for these veins of power, the fact that they exist is a common trend, even in modern mysticism traditions. they are usually invisible, though those with the sense can detect them. In some cases, the lines can be colored by the emotions of humanity, in other cases they're pure. In either case, being trapped without a line near by is recipe for disaster.
Drawbacks
The third part of any magical system, and the part that I'm having the hardest time working on. It is, however, probably the most important. When you introduce magic into the world, there's always a price. Some of the most interesting ones I've come across recently are: Sanity (Magic or Madness), casting magic eats away at your mind, causing you to slowly lose it and Memories (Magic in the Blood), every time you cast you loose a memory. Common ones include energy, and control. I think I've read one where lifespan was the cost to pay as well. Control is an interesting case, because it relies on external (or sometimes internal) policing. The line between white and black magic is thin, and all to easy to cross. Crossing it means that your own kind will hunt you down (Rouge Mage, The Hollows). Either that or you give yourself up to a non-practitioner (Rouge Mage, if memory serves. Also some aspects of Bitter Night are similar). If you see references to Mana, then that's a toll of energy, of life force, which is usually fairly hard to come by.
Conclusions
So I didn't get as much commentary in there as I wanted, but it still got the point across. If you're looking for small magical effects, then keep it only to the level that it services the plot. More active magic needs three basic components for an interesting story: a method of control, a source, and a drawback. The drawback is what provides the drama, and to an extent the realism. The source is usually simply thematic, but sometimes there can be mroe to it. And the method of control helps you define a progression (learning spells, learning words, etc.), to allow your characters to grow.
Books mentioned
Wayside School - Lewis Sachar
Indian in the Cupboard - Lynne Reid
Dresden Files - Jim Butcher
Harry Potter - J. K. Rowling
Name of the Wind - Patrick Rothfuss
Eragon - Christopher Paolini
Earthsea - Ursula K. LeGuin
The Belgariad - David Eddings
Dragon Nimbus - Irene Radford
The Hollows - Kim Harrison
Meredith Gentry - Laurell K. Hamilton
School for Adventurers
Mortal Instruments - Cassandra Claire
Mathemagics - Margaret Ball and Larry Elmore
Wizardry Compiled - Rick Cook
WebMage - Kelley McCullough
Weather Warden - Rachel Caine
Warrior and Witch - Marie Brennen
Darksword - Margaret Weis and Tracy Hickman
Magic the Gathering: Whispering Woods - Clayton Emery
Dragon Ball Z
Circle of Magic - Tamora Pierce
Magic or Madness - Justine Larbalestier
Magic in the Blood - Devon Monk
Rouge Mage - Faith Hunter
Bitter Night - Diana Pharaoh Francis