I have experienced this since I was a kid...not so much since I hit my 30's, but it's still just as freaky as it was when I was little and thought something had literally seized my soul. I find it happens more often when I nap, rather than regular night-sleep.
I stumbled over the name Sleep Paralysis a few years ago, and since then, it has been so much easier to deal with-knowing that I am not alone in having the experience.
Even though it sucks to be "aware," yet unable to move or speak, I can think to myself "this will be over in a minute...you are sleeping, and there is nothing there..." it definitely helps to stay calm...sort of manipulating it like a lucid dream really makes it less freaky. I think that may be why I don't have as many episodes as I used to.
Another good explanation is here:
http://www.stanford.edu/~dement/paralysis.html