This isn't specifically for piano, but more for musical composition in general. Every now and then I try my hand at writing a piece of music, and I have no trouble making a tune and variations to it, but what I really have trouble with is making the rest of the song, that is, everything else besides the chorus/main-theme. Therefore whenever I write a song, it always turns out to be some sort of fugue or canon. I can't seem to figure out how to make those alternate tunes that always seem to appear between choruses. Are there any compositional principles that apply to the construction of this part of the song? Any tips or tricks on how to go about thinking them up? Thanks for your help!