Here's a tip from one of the best that I've passed on to my band's main lyricist.

On a TV special, they followed Mick Jagger around for a while with a camera, and he described his procedure for writing lyrics. Basically, he writes a whole bunch of prose (i.e. ordinary writing, without a poetic quality), as the inspiration hits, in a small notebook he carries with him everywhere. Then, he listens to whatever Keith or whoever is writing the music side of it has for music, and he'll review that for a while, until something he has written in his book seems to fit and he'll begin improvising lyrics based on the ideas in his book.

This method is pretty good, and our lyricist uses it almost exclusively now.