How to keep sharpie on the back of my lifeproof iphone 4s case?

How do I keep sharpie on the back of my lifeproof iphone 4s case?

So I got my black lifeproof 4s case signed by John Wall. He signed the back of it in dark blue and I want the signature to stay on the back and not fade. I'm planning to still use the case. What should I do to make the signature stay on their? If so, should I still use the case?