All I gotta say is make sure you have two things on hand:
1) A TRUSTED tripsitter. Not only trusted, but experienced with psychedelics and handling other people's bad trips well. This is pretty crucial if you ask me when first trying heroic doses of any psychedelic. It could mean the difference between a beautiful experience and a terribly ugly, bad experience.
2) Benzos! Preferebly xanax. Xanax has a quick, quick onset (15-20 minutes). If the trip turns bad, even first swalling 2-4mg might be comforting before the effects even kick in. (I say 2-4mg because I have a tolerance. Myself, I'd probably go quite a bitter higher in dose because of this tolerance. For someone with zero tolerance 2mg would proabably be sufficient. However, because of the high mushroom dose a higher dose of benzo's may be required to calm one's self down from a frenzied state).
Be careful my friend. The first time I ventured into the mushroom realm I was hippy flipping and handled 5-6g pretty easily. I'd attribute the easy-goingness of my experience to the fact that I was rolling. It definately took any sort of edge I might have experienced off, but I had no idea of the actual potential of the mushroom's true power. I found it out on my second trip on a half ounce (apx. 14g), this time without any rolls or any benzos handy. Probably the stupidest dose and event I've ever put myself through. Although, I'm glad I was shown what I was shown. For a good year or two afterwards I couldn't trip on mushrooms without recalling the experience literally, no matter how low the dose. I finally got over that with the help of LSD and some deep thought/meditation. BE CAREFUL and consider those two things I listed! Godspeed.