I write it down on A4 paper, often with diagrams. When I have time to develop the ideas, I make few skethches some of them to be thrown away. I put them to a r

I write it down on A4 paper, often with diagrams. When I have time to develop the ideas, I make few skethches some of them to be thrown away. I put them to a ring binder together with project notes, specs etc. Works well with note block papers with holes or classic papers & hole puncher. Ocassionaly I review the content of the ring binder, throw away outdated stuff and make note of forgotten ideas worth developing more. I like Feynman problem solving strategy: keep two or three problems in back of your head and when you encounter interesting method, try applying it to the problems you keep in your mind.