I use my phone to record, and go for a walk in the woods and hum what I've written so far.  I always end up spontaneously adding something new, or varying the rhythms, or putting in a key change, without really thinking about it as I go.  There's something about walking and getting away from the paper that seems to loosen up my brain, and then I have a record on my phone that I can go back through when I'm home again to remember what I came up with and write down the best parts.  Driving seems to work well too.  Something about being in motion.  There is a need to fill the aural space with sound, and the brain obliges.  (:  But for me, the most important is a way to record, so that I can just hum, without trying to write it down at the same time.  It seems to free up my brain to let the music out.