I just drive slower to reduce noise. When it comes to cheap and effective, it's gotta be high on the list. I can't believe when I was younger/stupider when I drove so fast that I would just turn up the music to cover the noise! Gah!
Driving fast + louder music = bad FE. No wonder my earlier FE was a pitiful 33mpg.
I can't comment on sound absorption techniques as I'm too cheap/lazy to do them (and if I wasn't so lazy I'd pull out my rear seats and that will only make the noise levels inside worse I think). Plus at 40-50mph I don't really seem to mind the noise.