Have a look at this URLs:

http://en.wikipedia.org/wiki/Classical_element

http://www.thedance.com/wicca101/4elem.htm

The technique mentioned (ie, to meditate about the elements in our daily lives) is taken from the books of an author called Ophiel (from his series of books "The art and practice of...").

Two of those books are "The art... of Astral Projection" (where a LD method is given), and "The... of occultism". He gives a very good insight of how the astral is. I think they're a must, but that's just my opinion.