Easy: you hit with the part of your shin that's directly under your knee wich is mega-hard. The nerves in your shins go numb/die off very quickly along with the hairfolicles.

Don't forget there's a lot of technique involved as well: when you block a low kick with your shin you don't just lift up your leg but you twist your leg outward and curl up your toes...try both and see wich one you like best