I've got lots of things lying around, from finished verses to just one or two lines, or sometimes even just a potential song title. I'm sure many other people here do too, so what techniques do you use to take the base idea and flesh it out into an actual song?

This is something I've been having trouble with for quite a long time. I have tons of 'seed' ideas but most of them don't develop into anything because I just can't find any way to make progress with them.
To me, it's simple. You just do it. Plenty of people on here will go on and on about theory or whatever, that doesn't matter. If you have audacity or some recording program, record a guitar track that matches up with the song structure you have in mind. It doesn't have to be perfect. Then just record a vocal track over it. Try different melodies and experiment. Even if it's not in the right timing, who cares. Just do it. The more you do, the more you'll hone your skills anyway.
Every so often, I take those snips and verses and titles, and try to put them together into something. I am not well versed in theory, so I just play my guitar as best as I can match whats in my head, then draw out a crude tab. I'll tweak the words to match the music sometimes. But, as Rocker and Nike says; Just Do It.

I've still not recorded anything, because I'm fairly certain that Axle Rose is a better singer than me, and I'm not homicidal enough to subject anybody to anything that manages to be worse than Axle Rose's singing. Plus, I can't seem to completely work out the distortion and muddiness out of the guitar tracks when I do record. Of course, using a $10 headset run into a laptop soundcard is not going to get the best sound.
I meant more in terms of lyrics than the musical part of it. Thanks for the replies though.
Everyone has their different methods, there's no easy way to explain it.

For me, it's humming out a vocal melody to a pre-written track of mine, then splicing together lyrical ideas until it fits that vocal melody.
Great question. I approach it like a research paper. I scribble a boxful of scraps, lines, phrases, hook ideas. Then, I spread 'em out on the desk. Often, these scraps with self-organize into three or four distinct subjects. (maybe, 'I hate my neighbor,' I'm feeling sad' or 'I really like so-and-so's butt)
Take one pile/subject and toss out the least interesting of the redundant phrases. Then, determine your 'valedictorian,' your big hook/title. Now, sing your hook over and over til it finds it proper and awesome phrasing. Once your hook has proven itself, you now know that you have a song worth writing. Now, start really crafting your verses so that they make your case/build, etc.. Hope this helps.