Spiritual shapehifting is a powerful tool for change in conciousness. It should be used with caution, and care. Most people, when they think of shapeshifting, think of people sprouting hair under a full moon and running rampant over some poor hapless town, devouring anything in its path. Shapeshifting in shamanic practices all over the world refers to assuming, or aspecting, elements of the animal invloved - this may be a manner of movement, speaking or vocalizing, mentality or any number of magical practices. In true fashion, over time humanity has begun to define shapeshifting as a physical thing only, being unable to concieve or believe in something they cannot actually see. Shapeshifting does not always have to occur in the physical plane, though, and 99.9% of the time doesn't. Anyone who can meditate well or travel astrally can shapeshift, with a little practice.
The theory behind much of magic is mind over matter, and this is true for shifting as well. The method that I describe here hinges on the ability to meditate deeply, and astrally project. I don't recommend any form of astral shifting work to be undertaken by anyone who is not an experienced astral traveller or accompanied by one. However, this isn't to say you can't learn. I simply recommend that you first learn the realms of the spirit before trying to alter yourself.
To get ready to attempt a shift, I suggest you work in a circle, or someplace you feel comfortable or safe. If you want, light a few candles and some incense, play your favorite music softly, have something you really like with you. Just be very relaxed and comfortable in your workspace. If you're uncomfortable, it's not going to work. If you have any spirit animals, or spirit helpers, talk to them.
Spirit helpers, totems and familiars are great for helping first-time spirit or astral shifters, as they can help guide you into a form that's right for you. Maybe they have a preferred other-form that you could use. (As one of my totems is Raven, I frequently use that form.) Anyway, get yourself set up and comfortable.
Get well grounded; send that root of power down into the earth a good piece. If you're not well grounded, sometimes you can get curious overlapping of what you see as a spirit, and what you see as a human, and it is very disorienting, not to mention disturbing. Center yourself well, keeping your power in a nice ball, and focus all of your power into a fine point. The best example I can give is that of a flashlight....the more focused the source of light is, the sharper and more precise the larger circle of light it gives off is going to be. If you stay well focused, then getting out of your body is going to be that much easier. Use whatever power is most familiar with you: Earth Power is the most readily available, but you can use a node of natural power, take some from a holy place, or use your own. If you want more information on centering and grounding, go here.
For those embarking on a spirit shift, once you have centered and grounded well, begin to deepen your meditational state. Feel yourself lose the rest of the world, and meditate on the Spirit Animal you are trying to shift into. If it is a Totem or other familiar Spirit Animal, meditate on the aspects of the animal you may admire, or desire. Meditate on being of the same species as your totem.
If you wish to shift into a slightly more unfamilar form, I suggest you treat this the same as working with an unfamiliar God or Goddess. Do some research, get to know their habitat and physiology and their temperament, and then meditate a few times with the Toteml in question; this will prevent the spirit from being angered at you borrowing its form or imitating its Children.
Ask your spirit guide or totem to help you for the first couple of times you shift, because what you are really doing is "aspecting" that animal - mentally and spiritually taking on aspects of that creature's body language, mentality, senses, and ability. You may have the sense of having your Self merge with the spirit of that animal - that's normal. You may also feel that you have other "parts" - ears, tails, paws, snouts, etc. that aren't yours, but are. That's also normal - we call those phantom limbs. Your guides or totems will generally help you learn how to "be" in that shifted state - you may notice vision or hearing or olifactory changes, desires that seem odd to a human but perfectly normal to an animal, or the need to be somewhere outside.
This is classic shapeshifting in a shamanic sense.
If you are embarking on an Astral shift, after grounding and centering deepen your meditational state and use whatever method you prefer to achieve the state of vibration where you can leave your body. When you feel focused and kind of floaty, in that sort of waking-sleep state, leave your body. This is exactly the same as if you were just astrally travelling normally - you could at this point go anywhere on the astral plane. If you are unfamiliar with astral travelling, please go here.
When you are outside of your body, there will be a fine cord that connects you to your body. This is the same as any other astral experience - DO NOT SEVER IT. It is the only thing that keeps your soul and your body together. It will stretch and grow along with you for however far you decide to go, but if you sever it your body will be left without a soul, and a body without a soul eventually dies unless hooked up to all sorts of hideous machinery. It lacks a purpose and a will to live, and you'll be left wandering the earth without the means to the Underworlds, save by the good grace of the Gods. This is why I say that you should have some experience in out-of-body magics; it makes it less dangerous.
Once you find yourself on the Astral Plane, I suggest you either go to your sanctuary, a safe place you have created for yourself, or a safe place of another's whom you trust that you have permission to use. It's the same as the protective place you have around your body on the physical plane - just a good precaution.
When most of us start astral travel, we always keep a semi-true mortal form - we are our Self as it is in pure energy. But it's extremely malleable - if you can focus your will and intent, you'll look like whatever you think you look like. The trick to shifting is to think of yourself as something else. Use your Mind's Eye to shape the most accurate, detailed image of whatever you wish to be. Let the power that flows through you flow into it, filling it and charging it. As you do so, imagine yourself taking that form, feeling your hands become paws or wingtips, feeling your feet change, your face. When you don't feel anything else changing, or you've completely filled the image with power and it can't hold anymore, you should be the image you've created.
To reverse the shift when you're done, take the image and "undraw" it, removing all the power from it first. Once the image is undone, you will be a thoughtform, a mass of energy that is your Self, but hasn't been given a specific form. Think of your Self again, your natural form for the Astral, and let that become your Mind's eye image again. Fill it with power, and gradually unfocus the power's projection. You should return to your body, if that's what you're aiming for. To simply undo the shift just remove the power and undo the image, then redo it to be whatever your next form needs be if you're still remaining a spirit-form. You will always be in the same form on the Physical plane, no matter what form you have taken on the Astral. Some people will find that their trueform on the Astral isn't humanoid at all - life's full of surprises. ;)
Whichever method you choose to explore, spiritual shapeshifting can add a lot to our everyday lives. Use it responsibly. :)