The Law of Contagion: Once together, always together; or The part of the thing is the whole of the thing: A part of something - like one of someone's possessions or a part of their body - can be used to work magic on it over a distance as though it were physically present. The more intimate the connection the better the link works so a discarded cigarette packet is little use, whilst a favourite shirt or piece of jewelry is better and part of someone is ideal1.

The Law of Sympathy: Like attracts like or The image is the thing: Where a contagion isn't possible an image of the target may be used instead. The better the image, the better the link - and even better if the target has personally endorsed the image. Signed photos are really good for this.

The Law of Consent: No injury is done to the willing or this works a lot easier with your co-operation: Depending on who you ask, this is either about the idea that magic is said to have difficulty effecting those who do not believe in it and/or about the idea that it is possible to consciously resist magic and it is far easier to operate a working upon someone who consents to be targeted than someone who doesn't.

Equivalent Exchange: Everything has a price: Magic conserves a given level of value (although one or both "parties" may have different ideas of how something is valued) and a given effect must be paid for with something of at least equal worth. Part of the fun of magic where this applies is making absolutely sure you understand what you are paying before you seal the deal. This also tends to lead to magic that takes the path of least resistance, moving stuff about rather than creating it and what have you.

Reciprocity (aka. "The Law of Triples"): Whatever you do by magic, for good or ill will be repaid to you threefold.

Reversal: Whatever magic does, magic can undo: The idea that anything created by magic can be dispelled or otherwise undone by magic without needing to be physically destroyed. Conversely, this also allows there to be space for anything killed by magic to be brought back under the right conditions...

Distortion: Magic changes nothing … permanently: The idea that magic works by bending the universe out of shape but does not actually change it - sooner or later the world will snap back into its original shape.

The Law of Balance: In the end, everything balances out: Very much related to the laws of equivalent exchange and reciprocity, this idea suggests that magic cannot create or destroy anything, but must move it from place to place (or time to time) - so good fortune now must be repaid by bad fortune later, restoring life to someone means taking it from someone else … that sort of thing.

Backlash: If you don't know where it's pointed, it's pointed at you: Once a magical working is started - especially a working of the harmful variety - it needs to go somewhere. There are likely to be ways and means of safely "earthing" an aborted working (if you can get them right and have the chance to do so), but more worrying still are those workings which miss their target and/or snap back because they are resisted or reflected. Death spells are especially notorious for rebounding on the caster if they fail to kill the target. Such traditions may well develop ways of passing the damage onto someone else - especially the clients of those casters performing this kind of sending for hire or reward. This is the sort of context in which you need to be absolutely sure of your targeting data before you start your attack.

Found here, and edited somewhat.