Advice from J. E Littlewood
The Four Stages of Creative Work -
Preparation, Incubation, Illumination, Verification
Preparation
Directed by the conscious - stripping the problem down to its essentials. Suveying all related knowledge and considering possible analogues keeping it in mind through all other work
Incubation
Is the work of the subconscious - the waiting time and it may be several years
Illumination
Can happen in the fraction of a second. It is the emergence of the creative idea. The rapport of the conscious and the unconscious.
Recommend walking and the relaxing occupation of shaving as helpful to the process of illumination.
Verification
The Four Steps  -
Understand the Problem, Devise a Plan, Carry out the Plan,  Look Back
Step 1. Understand the Problem
Understanding the problem is often neglected as being obvious. Yet students are often stymied in their efforts to solve problems, simply because they don't understand it fully, or even in part. Know what is asked, list the available information.
Step 2. Devise a Plan 
There are many reasonable ways to solve problems. The skill at choosing an appropriate strategy is best learned by solving many problems. List available information and possible strategies, 
Step 3. Carry out the Plan 
You need care and patience, given that you have the necessary skills. Persist with the plan that you have chosen. If it continues not to work, discard it and choose another.
Step 4. Look Back 
Much can be gained by taking the time to reflect and look back at what you have done, what worked and what did not, and with thinking about other problems where this approach and solution could be useful.
See details below.
 More on George Polya's Four Steps