This is actually the method I've used most often to get into LDs, and you're right, if you are already in sleep-paralysis it has a very high success rate. For me, it tends to feel like... I'm in sleep paralysis, I can't move my actual body, but I can sort of... sit up and walk away from my bed, leaving my body still lying in it. For some reason, I then usually climb out of my bedroom window rather than leaving by the door. I don't know, it seems perfectly logical at the time (and is a good way to start off flying dreams, I guess...)
I have two problems with it, though:

1. The part that I mentioned above where I sort of stand up and leave my body lying in bed (I've even occasionally turned around and "seen" myself still lying there asleep) is.. It works fine, it's is not a problem in itself, but - quite often it will be accompanied by utter irrational terror and a horrible creepy feeling of "if I leave my body just lying here, I'll never be able to get back in!" I know perfectly well that this is a ridiculous fear, but I guess my dreaming mind is not very rational, because sometimes it's so scary and overwhelming that I feel I have no choice but to try to wake up for real "before it's too late". Any ideas for getting myself past the irrational-panic stage?

2. This doesn't always happen, but sometimes when I do manage to get up out of bed (in the dream), I will find that my eyes are still closed and no matter how hard I try, I can't open them, and therefore can't see anything... I've resorted to trying to prise my eyelids open with my fingers sometimes, but no luck!