Desensitization to whatever makes you flinch. Surround yourself in it and have it go off whenever you least expect it. Over time, you will flinch less and less. However, this is a long, and sometimes it is forgotten by YOU, and also can set you back if you do it improperly.

Alternatively, you can remove the spinal cord. FAR easier. As you can tell, Old Manus removed his a long time ago. He is flinchless.