I usually don't worry about subject matter. I start with phrases that I like the sound of and string a few together. Those that don't seem to fit get kicked out and usually a basic theme will emerge from the wreckage. If I get stuck with it, I'll often put it away and forget about it.
Sometimes( and these are my favourites) I'll be picking through piles of ideas in my notebook or on sticky notes and find a long abandoned snippet that miraculously seems to fit another half-finished song I'm working on; a lovely happy accident. Then, of course, it's time to polish it and trim it down. As John Shuttleworth would say "Hone your lyrics!"

Steve Robinson