Hoodoo altar set for an uncrossing spell (from http://www.drkioni.com/)

One of the most commonly attributed characteristics of a witch is the ability to cast a spell, which is basically the means employed to effect some kind of change, to accomplish some magical action, to bewitch someone or something, to influence a particular course of events or to inject magic into healing remedies or objects. A spell can be muttered over a herbal brew, as it is usually portrayed in popular culture, but the best spells are done by means of elaborate and mysterious ceremonies of ritual magic.

The use of spells dates back to the Magi of Zoroastrianism and Ancient Egypt, which together shaped the Hermeticism of Hellenistic religion and the Greek and Roman mystery religions. All contain early instances of the use of "magic words" (said to have the power to command spirits), wands and other ritual tools, magic circles (to defend the magician against the spirits he is invoking or evoking), mysterious symbols or sigils (thought useful to invoke or evoke spirits) and spirit mediums. Galdr is an Old Norse word for spell or incantation, and it was usually performed in combination with certain rites.

Spells can be cast to affect the self, or directed to affect another person, and can be either beneficial or harmful. A positive spell is often called a “blessing”, although such archaic terms as “bewitchment” and “enchantment” are still sometimes used. Negative spells are generally known as “hexes” or “curses”. A “binding spell” is one intended to prevent harm, avoid danger, or to stop someone from performing particular acts (e.g. halting a murder, preventing a rape or even stopping the spread of gossip). 

A spell has a specific kind of formula, usually involving the use incantations, images and implements (sometimes called “charms” or “runes”) and a set of simultaneous actions, which are designed to gather magical power and direct it towards a specific purpose. It can consist of a set of spoken or written words, a formula or verse, a ritual action, or any combination of these. The failure of a spell to work may be attributed to many causes, such as failure to follow the exact formula, general circumstances being unconducive, lack of magical ability or downright fraud.

Voodoo dolls are used to represent an individual during spell-work in some traditions (from http://www.zombieye.com/)

Spells may be cast by many methods, such as by the inscription of runes or sigils on an object to give it magical powers, by the immolation or binding of a wax or clay image (poppet) of a person to affect him or her magically, by the recitation of incantations, by the performance of physical rituals, by the employment of magical herbs as amulets or potions, by gazing at mirrors, swords or other specula (also known as scrying) for purposes of divination, and by many other means.

Spell-work is largely based on ritual and often the simple lighting of a candle starts the ritual process. For such day-to-day spells as blessings, self-help, positive thoughts and the sending of helpful healing energies to others, it is not necessary to undertake the cumbersome task of a full circle-casting ritual.

It is important to know what phase the moon is in when practising spells and witchcraft. For example, it is best to do spells of a banishing or destructive nature during the waning of the moon, when it is dark and the new moon is about to wane. Spells of a constructive nature (such as spells for luck, fertility and protection) should be performed during the waxing of the moon, when the moon is full and bright. All spells should be practised while in a positive mood, because this will achieve the best end for whatever is wanted.

Within Wicca, all spells must adhere to the “Wiccan Rede” (the witchcraft code of conduct), meaning that any spells used to manipulate, dominate or control another person are forbidden. The other guiding principle is that of the “Three-Fold Law” or the “Rule of Three”, which states that whatever energy a witch or magician puts out into the world, be it positive or negative, will be returned to that person three times over (therefore, spells effectively bring about their own punishment or reward on the practitioner). Spells are generally used in Wicca to channel the power of the Wiccan God and Goddess (or sometimes other appropriate deities) in order to initiate positive change and for the resolution of problems.

Here are a few random spells from various sources. No guarantees are given!

• Spell Apple
  The Spell Apple is a spell for banishing a problem or a habit. The apple is cut in half sideways. One half is rubbed with mint while the bad habit or problem to be banished is said out loud. The halves are then rejoined with a skewer, tied with ribbon and buried. As the apple decomposes, the problem will go away.
• Friends Forever
  To ensure that a friendship will last for a long time, some cloves should be sewn up in two little bags, one being carried at all times by each of the friends concerned.
• Attract Love
  A sachet bag containing sage, rosemary and thyme should be kept in a drawer or under a pillow, or even worn against the skin. The bag must be moistened every seven days with seven drops of bergamot oil to be truly effective in enabling your heart's desire and attract love.
• Chinese Love Potion
  A traditional Chinese love potion consists of a handful of coriander seeds, crushed and stirred into a glass of white wine while chanting: "Warm seed, love run strong; warm heart, let us never part". The lovers then drink the wine, and their love is assured.
• Sweet Dreams
  A muslin “magic pillow” is made in which are placed red rose petals, honeysuckle flowers, powdered orris root, allspice, and a lock of the spell-maker’s own hair. The bag is then sewn up, and moistened thoroughly with oil of pine. When placed under a person's pillow, that person should dream only dreams of the spell-maker.
• Irresistible
  This is a love potion to be added to bath water, and is suppose to make the spell-maker irresistible. The potion consists of rosemary and thyme, steeped in boiling water inside an earthen jug, with powdered orris root and lovage root added later. This mixture is then strained and stirred before being poured into the bathwater.
• Money-Making
  This spell was used to literally make money. Twelve bits of banknote-sized paper are cut and placed in a box, with thyme sprinkled between each piece. The box is then tied with green string in 31 knots and buried seven inches deep. It was believed that if the box is dug up exactly one year later, it would contain real money.
• Banish Negativity
  To banish negative energy, place an onion quarter in each corner of the room overnight. The next day, wearing rubber gloves, chop the onions up and bury them. Repeat this routine for 3-7 days in a row.
• Cleanse the Body of Negative Energy
  First, clear the mind and light a white candle (representative of positive energy), speaking the following incantation: “Mother Earth, Fire, Wind, Water and Spirit, I ask thee to cleanse my body of all negative energies”. Then, light a black candle (representative of negative energy), saying the same thing. Then, light a green candle (representative of healing), speaking the following incantation: “Mother Earth, Fire, Wind, Water and Spirit, I ask thee to free and heal my body from all negative forces. Blessed be!” Then, sit back, keeping the mind clear, for fifteen minutes, after which time you should feel renewed and fresh.
• Spell against Nightmares
  To protect against nightmares, negative thoughts and “invasions of evil”, first gather three red leaves from any tree, plant or bush and lay them in a triangle on a flat surface. In the centre of the leaves, place an already lighted candle, and place a few drops of chrysanthemum oil on each leaf. Say this incantation three times: “Red leaves, gift from earth, Birth to death and death to birth, Keep all evil far away, Day to night and night to day”. Then, extinguish the candle, and wrap the leaves in a white cloth or pouch, which should be placed near your bed, within three feet of your head, to stop all nightmares and negative thoughts.