When I used to do planks I used to try to keep the natural curvature of my back but after doing lots of research I found out the a better way to do planks. When you perform a planks, you want to tilt your hips which will make you lower back flat. A good way to do make sure your lower back straight/flat is to put a book on your lower back and to try to make entire surface of your lower back touch the book. This quite straight forward. This position will activate your abs. This will make a world of difference and you will notice it instantly.