I just think of random things, and fixate on that for lyrics. If I can't come up with a good riff, I will put the lyrics into a melody and build a riff off that. If I have a riff I like then I will put lyrics to it and try to fit something to it. I don't listen to music at the time either, I think it's better to have a clear mind when writing. All the great writers of the past 60yrs all have different methods, ideas when it comes to writing. I think its just a matter of "doing it" and not thinking about it too much.