Sometimes, it helps to write out a couple sentences that tells what the song is about and where you want it to go--then just keep writing phrases that relate to the subject(don't worry about whether they rhyme or scan, or even make sense). After a while, you'll have a bunch of raw material that you can work with--there is a downside, because you can get hung up trying to fit a lot of odd pieces together, but that still is better than having nothing on the page--