Personally, I tend to write lyrics in an escapist fashion. So, much of the time, my lyrics are fiction based and I also tend to write from a light hearted standpoint in the fashion of Zappa, where he may address a serious issue, but do it in a comedic fashion.
Because of the fictional aspect I like to use, sometimes I find it helps to write out a story, then turn that to a song.
Usually I'll get an idea for one line and the rest will just flow. Sometimes I get stuck and I'll have to put it down for a few days or even months until the right time comes.
I guess using the method of a fictional story could work for you if you're short on ideas. Otherwise, to make it seem like a good prog song, just look up the biggest words you can find and right them down randomly so everyone will marvel at how deep and meaningful and artsy you are.