When I'd hang out at my ex's house when they were writing or coming up with stuff, what helped was just writing exactly what was coming to your mind at the moment and rolling with it. Don't worry about a topic, let it be as random as possible, and then after you've got enough lines, just edit it and change it from there.
That method produced:
"His follicles are laced with Bob Dylan
Mr. Tambourine Man I'm followin
Turn the key but the engine wont start
Actin dumb, but I'm feeling kind of smart
Live down the street from that Tony kid
Wanna hang out but he's kind of a skid
They always write the instrument parts first, though.