I've done it a couple times on accident. Didn't know what the hell it was until I started hearing about it. It doesn't help that I also have an incredibly active imagination and that state was just all too quick to jumping on any thought bubble I had. Four winged birds, three headed leopards, gigantic, neon, mushroom-like trees reaching into the sky with floating islands. Rainbow colored, insectoid, cat-beasts prowling in pitch black night. Yeaaaahhh. Kinda fun in retrospect but, absolutely terrifying when you have no idea as to what you're doing.
Really, I think what it takes is making your mind tense enough to where you don't actually fall into a deep sleep while still hovering between REM cycles and full alertness. Sure, you can try to chant mantras to get focused but, really, once you've said a word enough it just becomes a sound to your subconscious. If you're one to fall asleep quickly, this may be difficult for you. I think the best advice I've ever seen on lucid dreaming is to get a timer that just beeps on a set schedule to keep knocking you out of deep sleep while still not being enough to cause you to snap the trance. The trick is to make your mind believe after enough beeps that, any second, there will be a sound to snap you awake but, to have had the timer set to have an irregular delay between normal beeps, causing your consciousness to think that there was a beep due to pattern and waking you up into the dream.
