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“”RULE ONE: Never fly out of upper story windows. Always take off from the ground. Just in case you might actually be awake. And watch out for power lines.
A lucid dream is a dream in which the dreamer realises that it's a dream, and can begin to control the dream. This feels pretty much like if The Matrix was real. The term was coined by a Dutch psychologist in 1913, but scientific inquiry into this phenomenon did not seriously start until much later. In the early 1980s, sleep researchers Keith Hearne and Stephen LaBerge independently discovered a method for lucid dreamers to signal their state to observers, proving their awareness during what's supposed to be a sleep phase. While the phenomenon has been established as real and is being studied as a valid field of sleep research, some New Agers have taken this as a starting point to make wild and unsubstantiated claims about the nature of consciousness, and that it might even be key to unlocking parapsychological powers.
 How-to guide
There are various methods to induce a lucid dream. This takes advantage of the fact that the "physics" of a dream are somewhat different to real life. Once you realise you're dreaming, jump and try to fly, walk on the ceiling, or go ahead and do all the cool shit you wish you could in real life.
- Hold your nose and try to breathe. If you can still breathe, you're dreaming.
- Try switching on a nearby light switch. If it doesn't work, you're dreaming. In dreams, mechanical objects don't quite work as they do in reality, so driving a car will be different, for example.
- Look at some text or a watch and check the time, look away and check it again. If it has changed, you're dreaming. Dreaming doesn't stop you reading but it does stop you making out letters as sleep shuts down certain centers of the brain - essentially turning you highly dyslexic.
Just in case these tests give you inaccurate results (a mind while dreaming will attempt to rationalise and self correct these inconsistencies), it's always a good idea to start flying from the ground, not a window or tall object - just in case you aren't dreaming. These techniques do require some practice, you need to get used to doing them regularly so that in the dream you start doing them automatically.
Your chances of getting into a lucid state are also increased if you can recognise that it's a dream without trying the above tricks - or at least you're less likely to rationalise the answers and stay in a non-lucid state. This can be done by keeping "dream diaries" to aid the memory of dreams, which are often forgotten very quickly. The more about dreams you remember, the more easily you can recognise and react to them.
To take advantage of a longer lucid dream, you need to know when your sleep cycles are longest. No one immediately enters a deep, dream-like state, so the longest dreams happen towards the end of a sleep, early in the morning for most people. Even if you wake up part way through the night, you will slot back into the rhythm if you fall asleep again. To take advantage of this, it works well to have 6 hours normal sleep and wake up for half an hour or so while constantly thinking about lucid dreaming. Then when you fall asleep, you'll be more likely to recognise the dream-like state and the lucidity will last longer.
People who believe that dreams have some sort of predictive nature are usually against any activity that prevents them happening naturally. Of course, dreams have no predictive ability beyond the normal amount of chance and shoehorning ability of the dreamer, so dream away - just seriously make sure you really are dreaming before you do something dangerous. While this method has its uses as a stopgap measure until we've got a working holodeck and may even offer some of the benefits of meditation, it's important to note that there's no evidence whatsoever that you can use it to unlock any secret powers hidden in your subconsciousness. Flying and such may work while within the dream, but nobody should expect to gain special powers in real life.
 See also
- Lucidpedia - Would you like to make your dreams come true?