I spend almost all of my free time just writing music. It isn't always good, by any means. But to become good at composing, it's something you have to do almost every day.
Sure, I (as well as Silverline and arlen as well, Im sure) come up with the random short unfinished bits too. But, those are good as well, because they can become useful later on when you need an idea for a little passage in a song.
I don't know about arlen and Silver, or any other composers who may be here too, but I find if I just sit there and tinker around for a while, a fairly decent idea usually comes out, and then I can move on from there.