Personally I've never found the need to delve into the mechanics of writing lyrics - I just write when the mood takes me and if it comes, it comes (and usually without too much effort). If it doesn't then I just put it aside and maybe come back at it another time to finish it off.
As (most) people on this forum know by now, I'm not a huge fan of rules when it comes to writing - I feel it can stifle creativity (at least for me) more than help it. Not that I don't occasionally lock myself into a set pattern or structure - I just find myself, more often than not, breaking it, and realising I've got better lyrics for it than if I'd have tried to retain conformity.
I know what you mean about unfinished lyrics though. I've probably got a few hundred lying around (and a number of them probably actually quite good or good ideas I'd like to revisit at some stage). I suppose at the volume that I write it's not such a big thing to me, though, not finishing some lyrics.
My favourite songwriter is Tuomas Holopeinen (of Nightwish). If you ever read any of his lyrics or heard any of his songs you'll realise that (a) the structure is whatever the hell takes his fancy and to hell with the rules, (b) half of it doesn't rhyme or even come close to rhyming, and (c) when you listento his songs you realise none of that actually matters because they end up working. Basically, he'd rather make the music fit the words than the other way around!