Well, I consider this to be a very interesting topic, and also a very fascinating one. I think there's a lot of unanswered questions about this, and a lot research to be done, and, of course it's not always easy, considering the subjectivity of the experience. Anyways, I'll describe some of my experiences with Astral Projection.
When I was 15 years old, I used to enter sleep paralysis a lot. I didn't knew anything about this so it was terrifying for me because of the strange sounds and the creepy voices that I heard frequently during this situations. During this time, my experiences of sleep paralysis were very common, sometimes it happened to me two or even three times a month. Eventually I experienced this sensation of getting out of my body, but I couldn't see anything, it was like if I was blind, I couldn't see anything, everything was black.
The most common characteristics of an Astral Projection for me are:
-Blindness: After I'm out of my body, I can't see a thing no matter what I do. Everything is black, just darkness. I just can feel my "astral body", but just as I said, I'm totally blind.
-Pain: Usually, before getting into an astral projection/out of body experience, I feel a really painful sensation in my chest. Sometimes I felt like if I was about to die. I have experimented with this, letting the pain to keep growing to see what happends, but then it expands to my arms and legs and it gets more difficult to resist.
-Slow and Difficult Movements: After I've gotten out of my body it gets really difficult to move. Almost impossible. I just feel that I'm there, floating, but I can't move, or if I can, it's really difficult and extremely slow, so I've never really been able to move enough to get out of my room.
As the years passed, the frequency of my experiences decreased, so in order to imrpove them and to induce an astral projection, I started to read articles on the Internet looking for information about this, I even bought a couple of books about parapsychology and hypnosis. I decided to practice this so I could be able to induce sleep paralysis and to astral project at will. Personally, I'm conviced that you can induce this experience by educating yourself about this and practicing. It's not impossible, just difficult, and requires initiative, a lot of dedication, effort and self confidence.
I tried to practice this, but, I had to go to school, homework, and many other things, so there wasn't enough time to train and research this by myself, and, for some time I couldn't do much. However, the astral projections didn't stopped.
Now, let me tell you about my most memorable experience with astral projection, the most awesome one I've ever had. This one happened to me a year ago:
I went to sleep, it was a normal night, a Friday night to be exactly, so, eventually, I fell asleep... Then, suddenly, in the midle of the night, I woke up... And I was totally paralyzed. I couldn't really control my thoughts, my mind was very blurry. I remember that in this ocassion I didn't heard any creepy voices or sounds, everything was just totally silent and calmly, and I felt like I had my eyes opened and that I was looking my room, but it was just an hypnagogic hallucination, then, everything turned black in my vision and I guess I went to sleep again. Later on, in that same night I got out of my body, but I never felt when it happened... All of a sudden, I found myself there, conscious, I wasn't blind, this time I could see, I wasn't blind like on my past experiences, so, I was there, floating in front of the small table of my room, I could see the window curtains, and my hands in front of me. I noticed that my skin color wasn't normal, it wasn't clear, instead, it was a dark blue color, and my fingers looked almost like if they were claws. I wasn't scared of anything like that, neither I felt afraid of something, in fact, I was very calm, and comfortable, for some reason I felt protected and happy, and I had a sensation of total weightlessness. This lasted for around 1 to 2 minutes until my vision fade to black.
The next day I was impressed by the experience. It felt so real and definitely not like a lucid dream... All I can say is that, that is something you just know when you experience it.
I still have to learn more about this, practice a lot, and do my own research. Maybe on the next days, considering that I will have a lot of spare time and that I could make use it.
Finally, I just would like to say that, while at first most of my experiences were scary (mainly because I didn't knew anything about this), later, they became more interesting, and, even if painful at times, at the next day I always felt satisfied... Satisfied for knowing how close I were to getting out of my body and exploring a whole new world.