I was just reading Paul Slocum's website. He's a musician who
makes music mainly with an Atari 2600! Anyway, he had
a really good tip:
"Keep a keyboard handy while composing. I keep a Casio SK-1 next to my computer."
Obviously, most of the time you won't have an SK-1 with you when you're using a PSP, but I still think it's a good idea to think about!
btw, here are the sites: