How can i wake up to quiet noises easier?

how can i train my body to wake up to something as quiet as a twig snapping in the forest or someone trying to walk up on me while i sleep? right now it takes a bit for me to wake up... i was camping once and my father shot a 12 gauge shotgun just outside the tent and i didnt wake up... if something serious was to happen i would never know.... oh and i dont use this yahoo question thing often.. at all really so can u tell me how to reply to a comment too XD