The ritual structure used by the vast majority of modern witches begins with the cleansing and definition of ceremonial space. This usually, though not always, involves the ritualized designation of the working area and is usually referred to as "Casting the Circle". In short, this is done by psychically manipulating one's personal power so as to envelope the working area and the participants involved. In a group setting this is done by everyone present, although more often the priest/ess or other representative will take the active role, acting as a unifying channel for the group's energy. In our tradition, we cast a circle for various reasons. The first and most obvious even to skeptics would be the psychological effect of removing ourselves from ordinary awareness. This serves to allow us access to our deeper selves, from where the power comes. Additionally it can be used to contain the power raised in a ritual until the moment it reaches its' highest point, at which time the collective force is released. It can also be used for protective purposes, acting like a barrier against negative energies, spirits, thought-forms, etc. I find it necessary to add that a ritual circle is NOT a requirement for the wielding of magick. I seldom cast a circle prior to spellworking, but prefer a circle to be cast when gathering in a group, or for more 'ritual' activities, such as the celebration of a Sabbat, or another special working.

When we walk the perimeter of our working circle we, if properly done, have entered into a type of trance that enables us to merge with, and to some extent control, the universal energies around us. It is in this state that we channel our energy (and the energies of other working participants, if present) into forming a psychic boundary that defines the ritual area.
 

Below, I give a very basic formula for the casting of a circle. Practice with it. See how it feels. Feel free to elaborate on it, or even change it completely, at your discretion.

First, walking widdershins (counter-clockwise) around the circle area, begin to sprinkle some salt or salted water to purify the area. Visualize and feel this as a cleansing fluid that transforms any negative energy, thoughts, or emotions, into a positive working force. You may wish to say something like:

By the powers of earth and water,
be cleansed!
Debris of spirit, senseless doubt, unruly thoughts,
Begone!
You are banished from this place.
This is my will, So Must It Be!

Now that the ritual space has been purified (with earth and water), we can begin to set up the circle. The circle itself need not be marked out on the floor, although many find this an enjoyable method. Light some incense (element of fire and air), thus purifying the circle-space with the four elements.

Beginning in the North (Different traditions vary on this. Feel free to use your own judgment.) Holding your athamé, blade outwards, (or if you prefer you may simply use your hands) begin to cast the circle by visualizing a blue flame, emanating from the blade, as you walk deosil (clockwise) around the circle area, and ending again in the North. (You might optionally wish to recite a poem as a circle invocation. There is an alternate invocation located HERE.) See and feel the energy as a band of vibrant power hanging in the air around the area before it expands into a sphere or "egg" enveloping the work space. You may then wish to say an affirming statement like:

The circle is cast.
We walk between the worlds.

Now that the boundary has been erected, it is custom to attune to the elemental powers. This is done though a series of visualizations, recitation of poetry, and perhaps singing or dancing. When the elemental power has been invoked, a pentagram (five pointed star) can be traced in the air as shown below:

It is visualized in the appropriate color representing the element and is seen as a portal into the pure energetic realm of the element in question. After this is done, the Goddess, and then the God are invoked to share our power with them and vice versa, charging the ritual with their energies.

It is important to point out that once the circle is cast, no one is usually allowed to enter or leave. It is generally thought that to do so would break the circle, dispersing the energy, tearing the boundaries. If someone does needs to leave, however, a doorway can be cut into the circle with the athamé on the perimeter, creating a portal. A pentagram can then be traced over the doorway to re-seal it.

The circle will usually remain until it is grounded or dispersed.
   

When the ritual is ended, the ritual space needs to be dispersed, returning the energies to their realms of origin. This can be done by several methods. One is to walk widdershins around the circle and "suck up" the blue flame back into the athamé, so as to 'uncast' the circle. Another is to visualize the energy as sinking down into the earth, where it is used by the earth. You may wish to experiment with methods of your own. One of my favorite methods is to first thank the elements:
 
 

We give thanks to (Spirit, Earth, Water, Fire, Air),
for watching over our rite.
Stay if you will,
Go if you must.
Blessed be.

Then the circle can be dispersed by raising the athamé to the sky, then pointing it down and touching the ground. Visualize the circles' energy rushing inward on your breath, sinking into your body as you open your arms wide and say:

The circle is open, but unbroken.
Blessed be.