The principle that magic can be invoked through likeness.
Cargo cults are an example of this. Cultists would build the likeness of an airstrip in the jungle, but no matter if they built pseudo control towers, radio and radar shacks, the planes never came.
Voodoo also rests partly on this principle, with the doll sympathetically representing a target person, although in most cases it extends the idea by requiring personal objects for full effect. These ingredients are also seen as being sympathetic to their origins.
Many elements of feng shui incorporate sympathetic magic, especially in its more mystical prescriptions for strengthening or weakening environmental influences. To be fair, I should also note that feng shui also has some pretty sound common sense stuff as well, however mystically expressed.
Sympathy has been called upon for medical results too, from Paracelsus to Francis Bacon, including treatment of wounds through applying powders and creams to bloodied garments or weapons. Ritual involving eating/burning/... the hearts/brains/... of your enemy is, of course, just more evidence of belief.