I dunno, Im not a successful recording artist by any means, but I rarely write songs down at first.
First, like Cyrokk was saying, I just pick up my guitar, and if I come up with something I love, I'll remember it the next time a pick it up. It becomes a part of my temporary repetoire; if it's not that great I usually dont remember it--maybe later Ill rediscover it though. But there are nice little songs Ive came up recently that Ive actually remember from the nite before.
Such as, I take a lot of sleeping aids, and it still takes me a while to get to sleep. Sometimes I start hearing music you know, and a I noodle with words with music Im hearing. I usually forget it, and for some reason it seems impossible to get up and play the songs-Im probably half asleep and dazed. But actually just a few nites ago I was albe to remember one, probably becus the nite before I decided to figure out how to play it mentally; yknow just a simple chord progression, bass line, the vocal line was catchy so I even remembered the basic words.-- Maybe this method sounds crazy, but I like it best. Though I wish I could remember them all!
I never finish words until it's time to record the song. And as Im usually guilty of doing, I dont listen to music in my car, I practice my repetoire. Mapping out lyrics, vocal style, etc.
I write lyrics down before a record, under the perhaps false belief of 'if I created it once, I can recreate it easily'. I usually give myself a few days to reconjure the lyrics, they're not so great so I really don't mind!
Though I havent recorded lately(my exband is dead), Im trying to find the inspiration to start the dubious task for my current set of songs.
That's my opinion, I could be wrong