I've read many accounts where songs just seem to "happen".

I think you can find inspiration from what interests you and what you are passionate about. Are there political issues you feel strongly about? Any particular news stories that had a strong impact on you? Plently of songs have been written in that vein. Do you enjoy reading or writing fiction? Science fiction? Early Rush and plenty of Iron Maiden songs have lyrics along these lines. Then there's Jon Anderson of Yes who would throw random words together just because they sounded good together but made no sense.

You could start out with a dittly like "I can't write no lyrics and got no inspriation" blues. Remember, bad grammar is a key ingredient of the blues :-). Somebody usually dies, gets arrested, or is wronged in some wayt too...