Personally I would lie about the coke and shrooms and play down how much pot you've smoked (say like five times or something). Stick to your story on the polygraph. Those things are notoriously unreliable, and I've heard you can fail parts of it and still get clearance. I think as long as you don't fail parts about whether you're loyal to your country, a terrorist, have committed a violent crime, etc. you'll be all right.