I have the best luck just grabbing a pad and a pencil and writing down anything that pops into my head. Listen to some music that has the same feel as what you envision your song like and see what happens. It's like a free association thing. Eventually you can put some of it together, add a few things and hopefully a song will emerge.
One of the best examples I have recently seen of this is on the “Some Kind Of Monster” Metallica DVD. The band was all sitting around with pen in hand jotting down lyric and music ideas. Some worked, some did not but when they put it all together a song was born.