or maybe write about a classic novel (you should write a song about The Time Machine), that seems to be all the rage these days
Again!? Goddamnit, people. It wasn't awesome when Maiden did it... Oh, wait. It totally was. Continue doing this.
I don't write lyrics, but that's mainly because I don't listen to lyrics. I am an instrumentalist through and through. I don't think in terms of words. I think in terms of chord progressions, voice leading and counterpoint (really basic counterpoint; it was the only theory class that I didn't do so well in).
However, I have seen lyrics get written before, and I agree that the way to do it is to write the whole thing, no matter how bad you think it is. If it's truly terrible, then you can do it again and nothing it ever a total waste. When forcing yourself to just write, sometimes an awesome phrase or even just a word occurs to you. You can take four or five drafts and lift stuff from each of them, and you might end up with the awesomest song ever.
Also, when writing music, I always find that the hardest part is starting out, but once you get the entire thing down, you can write a beginning that fits the awesome rest of the piece you've written. Who ever said that the beginning had to be the first thing you wrote?