This is an experiment that I suspected I may try based on my own experience and readings. I had a conversation with a fellow dreamer just a few nights ago where he gave me some tips based on his success with this very same thing.
Basically, at the point you become lucid within your dream, find a place to sit and meditate. For me, I picture finding a tree to sit under. Concentrate on a place in the real world that you wish to visit outside of your body. Then focus on pulling your astral body outside of your physical body and appear in the place you wish to visit. It is my thought that you are closer to the astral plane in your dreams, and entering the astral plane from a dream is far easier than to try doing so from an awake or meditative state. This thought was backed up by the dreamer friend of mine who claims to have had significant success doing it this way. At the time of this writing (12-Dec-2010), I have not actually tried it yet. At least not with intent to project/exit.
On your first attempts, the place you choose to visit is preferably a place that you are very familiar with and can visualize with a decent amount of detail. Also, some place where you can verify something you have seen on your astral travel by follow-up in the real/physical world.
For me, since my 5 year old son likes the light on at night, I have chose his bedroom as the first place to visit. I have read that when you are astral traveling or projecting on the lower (as opposed to higher, more abstract) astral planes that lighting may be in-line with lighting in the physical world. So, if you go to a dark place (like my bedroom) where the lights are off, you may not see as much as in a lighted place.
Each night before I go to bed, I am shuffling a deck of playing cards. I am randomly picking a card out of the deck to place face up just under my son’s night light. Of course I am not looking at this card. It is to be kept secret from me in the physical world. I read about this experiment from the book Astral Dynamics by Robert Bruce. Though I have altered the experiment to perform it starting from a lucid dream.
The experiement to perform is that the moment you are able to see and remember the card, say the card out loud. Bring yourself to wake up, if you can, and immediately repeat the name of the card out loud. Then go to the card to verify whether or not what you saw in the astral plane is true in the phsyical world.
According to Robert Bruce, he can do this with some regularity.