I use a white crayon, or a wax resists crayon. That way I can draw or doodle my image. For this example, it's a wild daisy, that I wanted the white to be the focal point in the image. To preserve the white I needed something strong and natural-looking. The white crayon can give you the look you are looking for.
Enjoy my short video showing how I use a white crayon or wax resists to save my whites.