As far as booze goes, i'm usually well advised to stay away from it before any important sessions or gigs. I get pretty messy, I can't do alot better in a rock context that I can comfortably pull off all the time other than a Keith Richards/ Johnny Thunders.
Thunders...speaking of drugs, now there's a guy who truly wasted his talent, so to speak. When he was on and sober, no one could touch that guy at what he did, and unfortunately, most of what we have is tenth generation live gigs of him trying to pull off the genius on "LAMF" live. I have a DVD, "In Cold Blood", where it's a complete trainwreck--he can't remember the chords, forgets lyrics, can't even keep his guitar strap on, and sings while on the floor at one point. Hey it's Thunders, but it really is kinda embarrassing and painful to watch. In that case, you really would have wished that he had a better set of friends and hangers on that didn't promote that stuff, because it affected him quite inversely. When you can't remember how to play the songs, it's time to kick the habit.