what I do is just jot down a cool line when it comes up in my head. I have a notebook I keep just for ideas. eventually you'll have pages full of stuff and you can piece together what sounds good or you'll feel productive enough to make an entirely new song on the spot. it doesn't matter if what you write isn't good. I'd say 95% of what I write is sh*t and I only end up using 5% of it. as long as you keep writing something good will come up.