One good idea is a method Mick Jagger uses; he carries a notebook around all the time and writes in it whenever he gets hit by the muse. Also, he doesn't write in verse, but in prose. That means that he doesn't write actual rhyming/rhythmic lines, but just thoughts in regular format.

I suggested this to my bass player/singer when he said that he was starting to run out of ideas for lyrics. It's actually worked very well for him.