Creating a piece of writing is not just a conscious process - provided you've already given it enough thought, even while you are sleeping, or doing other things, your unconscious mind is working on the plan, and useful ideas will come to you often when you are least expecting it. The problem is, you probably won't remember them, unless you write them down on the spot. This is why many writers carry a little notebook round with them, so that they can note down insights before they vanish.
If you are suffering from writer's block, it can often be a good idea to take a break for a while, and let your unconscious mind have a go at solving the problem. Just make sure you've got your notebook with you!