Good summary of a book by music historian Rosamond E. M. Harding, at Brain Pickings. Some selected quotes:
[M]any ideas outside the subject become associated with it by a kind of interest association and acquire a similar tone. Thus they tend to become available at the same time as the ideas directly connected with the subject itself. The variety of interests tends to increase the richness of these extra ideas — ‘fringe-ideas’ — associated with the subject and thus to increase the possibilities of new and original combinations of thought.
Harding’s method for capturing and harnessing ideas:
(i) The ideas occurring when in the glow of inspiration are (a) briefly noted down and (b) checked.
(ii) (a) The subject is worked upon immediately, the thinker being wholly absorbed by it to the exclusion for the time being of everything else, or (b) The subject is set aside to develop and is then worked upon after an interval of time has elapsed, © the first draft of the completed work or half of it perhaps is put aside to ‘mature’ for a while; then it is again revised before publication.
(iii) Working at two or more subjects concurrently.
(iv) Working up the imagination to the state of vision and sometimes an audition.
(v) Trusting to feeling (or intuition, instinct).
(vi) Procedure when baffled by a problem; namely, laying the work aside and turning to something else. This process may be repeated many times during the course of a long work of any kind.