I'd certainly agree with Sol about "use it or lose it" if you want to keep your range. Mine was at its widest when I was singing with a Gilbert & Sullivan company: you had to sing the notes that were written, couldn't drop it by 2 or 3 keys to suit yourself, so you did it, and it happened. And going back to folk music - don't shy away from songs with a big range: I usually sing to myself the highest and lowest note in any song before starting to sing if I am singing unaccompanied so that I can pitch it within my range. If accompanying myself or being accompanied by others, I'll make sure I've picked the right key.
I am slightly sceptical about some of the myriad of things that can supposedly affect your voice: obviously if you find something that works adversely for you, you avoid it: the same as any other food/drink allergies. I was a bit surprised to find that some of my singing friends would take a cocktail of port and sherry before singing if they thought their voice was a bit precarious - that and suck a Vocalzone.