Sometimes it seems like the songs that I write are already out there and my task is to find them. Ideas come to me when my mind is (relatively) clear, usually because I've done something athletic, explored my spirituality, read something meaningful and/or devoted myself to another person. Listening to others' music is always good, too.
Booze? puhleeaze!
Sometime the song starts with an idea (lyric-oriented) or a riff (music-oriented) - I saw a Paul Simon interview in which he said that in recent years he works more with chord progressions to get songs started; for me this doesn't generate the same quality, but then I haven't written thousands of songs like Paul Simon has - or sometimes both. Then I sit down with it and my guitar and try to develop the music. As I develop the rhythm and meter of the music I develop a framework for the lyrics. As I develop the lyrics I develop ideas for the music.
I try to keep an open mind and go where the song takes me instead of trying to fit it in to a pre-conceived notion of exactly what kind of song it's supposed to be. Sometimes the song seems to demand that I take it in unexpected directions. And although I try not to impose limitations, I try to accept that I am limited by my musical background and skills. "Play within yourself".
After a while (a day? six months?) I have a basic structure and concept and I try to write the rest of the lyrics and then refine everything. I once heard Nick Lowe interviewed, in which he said that after he writes a song, he locks himself in a room and plays it a hundred times, which has two contradictory yet essential effects: one, he knows it totally intimately; and two, the song has an existence all his own, almost like he didn't write it at all (akin to the "discovering the song in the air" idea I mentioned above).
I don't have the musical skill (or memory capacity) to just write a song all in my head. I need an instrument to provide structure. I think much more skillful musicians also feel the same way.