Practice this daily: First thing in the morning, when you're sitting down with your cup of coffee, tea, or diet soda, pull out a pen and paper.
Pick something in the room. Anything. It might be the coffee pot, or the mosquito that just landed on your arm. Write for ten to fifteen minutes about that object, in as much detail as you can. It can be accurate, or fanciful—but be as freely creative as you can be. Don't spend too long on this—you're not writing a song; think of it instead as exercising your creative brain so that when it's time, your brain will be ready to help you write a song.