Originally Posted by Zetetic

Sing! I know this sounds ridiculous, but if you're doing something that's contrapuntal or involves voice-leading, singing is by far the easiest way to realise the silent music in your head.

When I'm composing counterpoint, I first write out the part that's carrying the tune, then compose the bassline by ear; for the middle parts (which are the hardest to write), I sing along with Sibelius! This usually results in my writing a more convincing line that if I'd simply added the notes of the chord that were missing.