"The imagination needs moodling -- long inefficient, happy idling, dawdling, and puttering. People who are always briskly doing something and as busy as waltzing mice, they have little, sharp, staccato ideas...But they have no slow, big ideas."
That's a quote from Brenda Ueland's book If You Want to Write. Garr Reynolds at Presentation Zen has collected several other passages by her about creativity and inspiration, here and here.
(Thanks Nadine!)