A couple of tricks:

One(similar, at least, to what someone said above) is, just write down bits and pieces of the subject, whether complete or not, whether they even make much sense or not. I set out to write a sort of manual for newcomers in my field, and couldn't get anywhere when I was thinking of the whole project. But I would think, "Somewhere in there I have to say to do thus-and-so." So I'd write one or two sentences about that, and just let it lie there. Then I might say to myself, "Got to warn against blah-blah!" (which had nothing to do with doing thus-and-so). I'd write as much as I happened to think of about the dangers of blah-blah, and let it lie there. Then something else, and suddenly I'd think of an expansion on thus-and-so. Eventually, over a period of time, these odd bits coalesced, or they inspired a real flow of thought. And so forth. And eventually you get to the point where the odd thoughts need to be shoved around, rearranged to make a good flow of logic.

The other, in writing songs, is to take a song you admire, and figure out how it might have been written differently, or how the story could have ended in another way. Or how that comic song could be turned into a tragic one, or vice versa. Sometimes this works just as planned, and sometimes it jogs a completely fresh thought.

Dave Oesterreich