I remember a post you made awhile back, Eino, about the Imagination.
When it comes to composing music, I find that the Imagery/Imagination is one of (if not the!) best resource for practically anything. Couple the Imagery with music theory and you're set. The images our Imagination produces contain so much information--thoughts, feelings, will-impulses, sensations. All of this information can be communicated through music, with the Imagery, impulse, instinct, and music theory all coming together to say what needs to be said.
A simple exercise is to take a phrase and see where your Imagination goes with it. Something simple, like "The young boy lay on the grassy hill." An image may immediately come to mind, and perhaps even a story--everyone's own Creative Individuality will see it differently. This boy could be about 10, looking up, thinking about the red-headed girl at school he has a crush on. The hill is his quiet place, far away from everyone, with smooth grass and a big blue sky with huge white clouds. It makes me feel serene and happy, and I begin to empathize with the boy and his first love, and laugh at the sweet plight that he is in.
All of THAT being said, let yourself enjoy the image and trust your instincts and listen if you hear anything, hum to yourself, noodle on the piano--if it is strong enough, the image will affect your whole being. You can even change parts of the image by asking questions, like "What if the boy is pensive not because of a young love but because of a fight with a good school friend? What would that look/feel like?" A sadder atmosphere. Or "What if he is sitting there and a storm begins to roll in? Where does everything go from there?" Foreboding. The Imagination is limitless. Getting a lot of music theory down will help match what you think/feel/desire while Imagining. Imagining a dark, distorted hospital? Augmented and diminished chords everywhere (if we're going outside of traditional harmonic progression to create the proper atmosphere). The boy on the hill? Major scale, completely tonal, major harmony, with some minor seventh harmonies added in for a somewhat melancholic effect (that's just where my mind goes).
Sorry for the long rant there! The Imagination is just so exciting, as is pairing it with music theory.
Another cool thing to do is listen to world music and get a sense of what scales/modes/harmonies other cultures from other parts of the world use, and how it represents them--their location, their stories, what they think/feel, how they view the world, etc. Just some personal ideas/beliefs.