One rule I have is no alcohol while writing (caffeine’s okay, natch). And no other drugs ever. None of this tortured alcoholic author for me, thanks.
But once a year I allow myself an exception: One beer, one cup of coffee, then let yourself rip on the word processor. The combination of alcohol and caffeine loosens up the ol’ synapses in ways that can be amusing. Readers of The War of the First Day can probably identify the one scene in that novel that emerged from this method.
Actually, while I allow myself to do this once per year, I haven’t actually done it in three or four years.
You don’t want to make a habit of this, obviously, because you don’t want to become dependent on it.