A metaphor is a soft thinking technique connecting two different universes of meaning. Examples: Food chain, flow of time, fiscal watchdog. The key to metaphorical thinking is Similarity. The human mind tends to look for similarities. A road map is a model or metaphor of reality and useful for explaining thing, the Dolby Sound system is like a sonic laundry.
Excessive logical thinking can stifle the creative process, so use metaphors as way of thinking differently about something. Make and look for metaphors in your thinking, and be aware of the metaphors you use. Metaphors are wonderful, so long as we remember that they don't constitute a means of proof, as by definition a metaphor must break down at some point.
Imaging within another sensory or conceptual frame can help, eg. the visual images of spring which inspired Vivaldi's "Prima Vera", the dream that led to Berlioz's "Symphonie Fantastique," the art exhibition which Mussorsgy illustrated in "Pictures at an Exhibition," and so on.