How do I have an OBE?
 
There are several ways to Astral Project, some of which are specific to a culture or traditional upbringing. Shamans may call upon nature spirits to take them out of their body. A traditional Qabbalist may meditate upon the Tree of Life, specifically the path between Yesod and  Tiphareth. A Golden Dawn or Enochian ritualist may use scrying techniques and/or symbols.

Personally, I like a technique I learned from Denning and Philips more than a decade ago. It is a blend of 'overlapping' points from many techniques and rituals into a single process. Please remember that you may need to make adjustments so that the technique 'feels right.'  

Preparation

Before you try to astral project, know that you can do it, a process that may begin hours, days or even weeks in advance. Tell yourself that you will do this, and visualize yourself being out of the body already.

Make sure you will not be disturbed while you perform the meditation. Disconnect the phone any alarms (including the doorbell). Lock your cats away and kick everyone out of the house. Look around for anything that might be disturbing -- tight or restrictive clothes, bad smells (old laundry or a dirty kitty box come to mind as major distraction). Make sure the room's temperature is comfortable.

Some people recommend disconnecting all the electrical wiring in the room, although I haven't found it distracting. More importantly, remove any steel/iron/metal in and on your body - it can alter your energy-flows.

Go to the bathroom before you start.

Use music if it pleases you (it doesn't work for everyone).

Dim the lights to relax your eyes.

Learn the meditation properly by heart before you start.

If you are having trouble:

• cultivate a steady life-rhythm. For example, wake up and go to sleep at the same time. And yes, getting good sleep is vital. (Otherwise you'll relax, and just fall deep into sleep, never having a chance to go OOB.
• Get plenty of regular physical exercise, which releases energy blocks and relaxes the body.
• Watch your diet, stay away from spicy or fatty foods, and don't eat a lot before you practice. Many practitioners recommend not eating meat, specifically red meat as well.
• No coffee, chocolate, or alcohol. (I don't have to tell you to avoid drugs either, right?)
• Drink plenty of (pure) water, but not mineral water.
• A few hours before the meditation, open all the windows and  doors in your room and allow fresh air to blow through it.
• Pick a regular time to practice projecting.
  
  Keeping a positive attitude will make a big difference as well. If you KNOW you CAN project, you WILL.
   
  The Meditation

Lie down, with your head pointing to the East. Relax. Make sure you're in a comfortable position with your spine and neck straight. You may need a pillow to support your head. Breathe rhythmically. 

In, hold, out, hold, in, hold, out, hold. Breathe very deeply. Concentrate on your breath. Feel it flowing and blowing through your body. In, down, through your lungs, cooling your ribs, all the way down to your genitals, hold it, out, up through your belly, up your throat etc. Do this until you have reached a very relaxed posture and breathing rhythm. This may take a while, so don't rush things.

Pay attention to any disturbing factors, and consciously release your awareness of them as you relax. Tell yourself that they are not distracting, and then proceed to forget they are there.

When you are breathing rhythmically and are relaxed, begin to relax your muscles. Clench your toes for a few seconds, and then relax them. Feel a heaviness taking over your toes as they relax. Do this again. Repeat with all of your muscles, working from bottom to top. Relax your legs, your bottom, your belly, your back, your chest, your shoulders, your hands, your arms, your neck, your face, and your scalp.

Particularly important is relaxing your face! Try to really wear out your facial muscles. Stick out your tongue as far as you can, make funny faces, clench your jaw, lift your head a little of the floor, make yourself look like an idiot.
Enjoy the feeling of being totally relaxed. Try to relax your eyes.

Now comes the main meditation. It is made up out of four parts:

Energizing
Protection
Formulating the Astral Vehicle
Actual Projection

The energizing meditation is actually a very good meditation to perform to balance out your chakras and help you feel fit and clear. It's also very important to perform this meditation after every OBE session 9even if you didn't succeed), because it will re-balance you. 

On an 'out' breath, visualize the Crown chakra (the top of your head) as a sphere of brilliant, white light. Keep on breathing until you get a steady image.
 
On an 'in' breath, draw down a shaft of white light to your next chakra, located on your Brow (on your forehead, between the eyebrows). On an 'out' breath, visualize a sphere of dove-gray light on your forehead. Make sure you get a steady image. 
On an 'in' breath, draw down a shaft of white light from your forehead to your throat. On an 'out' breath, visualize a sphere of intense mid-purple light in the middle of your throat. Make sure you get a steady image.
 
On an 'in' breath, draw down a shaft of white light from your throat to between your breasts. On an 'out' breath, visualize a sphere of radiant yellow light in the center of your chest. Make sure you get a steady image.

On an 'in' breath, draw down a shaft of white light from your chest to your genitals. On an 'out' breath, visualize a sphere of Lavender, radiant and swirling in the center of your genitals. Make sure you get a steady image. 

On an 'in' breath, draw down a shaft of white light from your genitals to below your feet. On an 'out' breath, visualize a sphere of The seven prismatic colors (rainbow) swirling lazily below your feet. Make sure you get a steady image.

Once you have energized your whole body like this, concentrate on all the spheres and shafts at once. Be aware of all the colors. After having done this a number of times, you will feel actual tingling in the charkas as you energize them. Eventually you will feel your solar plexus (in the middle of your body, right between your belly button and ribs) throbbing like crazy. Here's where you will manifest your astral substance. When this happens, hold the spheres for a few more seconds, then let them slowly fade away.

Protection
Visualize a large sphere of radiant white light lying at your feet, until you get a steady image. Breathe in, and pull all that light in a long white shaft up through your body to the top of your head. See it light up the inside of your body, leaving a steady shine. On an out breath, let the light flow down along the outside of your body, encapsulating you in a waterfall of white, protective light. Let the light recollect at your feet and at an in-breath, pull it back up again. This is a greatly relaxing exercise, and it makes you feel great. Repeat this cycle for as long as you like, but at least 10 times.

Formulate Your Astral Vehicle
The astral vehicle is the 'car' your conscious needs to travel in. Here's how to do it. Visualize a jet of blue-silver mist to leave your body at your solar plexus. Make it form a cloud a few feet above you. Then form this cloud into a human figure, floating above you, facing down towards you. You can remodel this figure in great detail once you are projecting (although spending time prior to the exercise deciding how you want to look will help you achieve your desires). Once you are happy with this image, stop the flow of astral substance from your body, but keep a line of connection intact.

If you don't succeed in projecting, ALWAYS make an effort to 'take back' this astral substance (make the figure into a cloud and pull it back). This will prevent (unnecessary) severe energy loss and fatigue.

Actual Projection
Pick a point on your body. Any point will do, but I have found that the point that works best is your solar plexus (theories say that this is where your astral vehicle is attached to your body). Collect your being, your self, everything you are, all your wishes, thoughts, and movements into this point. Empty your head, your legs, arms etc. and recollect yourself at this specific point. Now visualize the same point on your astral body, which should be still floating above you. 
Make a mental resolution to lift this point where you are out of your body, along the line (cord) that links you with your vehicle, into your vehicle, to the same point in your vehicle, and SEE IT HAPPEN. Now the hard part: make a mental effort to 'turn around' in your astral vehicle thus come to face your own body lying a few feet away. Now expand your personality again, and make it fill your astral vehicle. This is rather like putting on a thick glove. Make yourself aware of your new body. You've done it! You can fly! Whee! Have a good time!

Important notes:
The first time it actually happens, don't do much more than just floating in air and looking at your body. Keep this up as long as you can, then slowly 'drift down'. You will likely automatically 'snap back' (this feels a lot like when you're lying in bed and get that feeling like you are suddenly falling down.). This 'snapping back' will happen more often that you wish for the first times you achieved actual projection. Don't worry. You have learned the 'knack', there's no reason why you cannot do it again, and you will learn how to stay out there for a longer period of time automatically.

Once you are projecting comfortably and you feel ready to explore, move away from your body as quickly as possible. This is because looking at your body, or getting close, may cause snapping back.

When you are back (or haven't successfully projected), finish with the energizing meditation, to revitalize your being.

I almost forgot: you need a good reason for your body to let you go. Any reason is a good one (including eavesdropping). Spending time prior to attempting to project, concentrating on your reason and why it's a good one.

Many people report a strange, whistling sound in their ears (or a cold draft on your forehead, a strange smell, or weird metallic noises). If this happens, don't be upset at it, it means you're on the right track. Thinking about your specific key-signal the next time you try to project may even facilitate the process significantly.

What can I do astrally?

You can remodel your astral vehicle with your willpower, to look like anything or anyone you wish. Your visual perception can be very odd - everything from feeling like you have eyes in the back of your head, to telescopic vision, to 360-degree perception. You can dress up as you wish, but research indicates that you cannot disguise your core personality. Other beings can see your aura very clearly, and will treat you accordingly.

Moving can be very difficult at first although travel takes places at the speed of thought. You move about by thinking about where you want to go or even just forward and backward. This may be awkward at first, but you'll soon get used to it. Don't be afraid about getting lost! Just think about your body or your room and you're there. Get a little closer, and you'll snap back automatically.

You can also move in time. This is a bit harder, and you may want to really practice before attempting it. To make it happen you can try visualizing a calendar losing it's days, or a clock going forwards or backwards. Some time periods are unreachable. In general, the farther you go back, the more the images you see will fade and become colorless. Generally speaking, more than 500 years back almost nothing can be seen.

When you go forward, images become more and more vibrant and sharp, until they become painful and unbearable. In either case it is extremely difficult to interact with the physical level when you are out of your 'home' time. You will likely also experience time elasticity. That is, spending a few minutes in the past may take an hour of your present. In general, the further you go back, the more time it takes in the present. The reverse is true for the future: in the present you may be out of your body for an hour, but spend hours, days, or weeks in the future time.

~from Maat's Book of Shadows