Start singing, you lazy swine.
by Andy Jenkinson
(Leicester, UK)
Singing, even if you 'do not' sing, gives you an extra level of polyphony when composing. Use your voice to make placeholder melodies, or words as you strum chords/play piano. Use your voice to suggest drum patterns. Bass lines. Guitar solos. Instrumental breaks. These parts - written using your voice will probably be far more 'lyrical' than if composed at a keyboard.
The melodic phrasing written this way is more likely to be continuous rather than disjointed - it is easy to sing. This has the added bonus of being 'catchy' to the listener - something that virtuous playing on the instrument would not be.
If you have the recording facilities try writing a chord sequence (on guitar/keys) and sing all the other parts. Notice how you tend to make 'hook' basslines and melodies and countermelodies / instrumental parts this way.
Now choose a nice instrument to play them on...
Et voila! (Or Viola!)