This is a skill that can be developed. One way to start is by using the techniques for inducing lucid dreams. This consists of programming yourself before you fall asleep to have a particular type of dream and to be in conscious control of it. You can then play audible or subliminal tapes on an autoreverse player all night to help reinforce this programming. It may be induced or spontaneous.
Many lucid dreams are actually astral projections.
Yes, the astral body does separate from the physical body during sleep. In most cases it remains close to the physical body. The astral body is not the same as the subconscious mind. Sleep programming tapes work on the subconscious mind and have nothing to do with the astral body. To a degree.