I like to hide in a ground blind and when a deer walks by I plunge a knife hand into his ribcage, rip out his heart, and show him his still beating heart just before he dies. then I spike his heart like a football and do an endzone dance!

If I have a bow I aim just behind the front shoulder and just a bit below center. Of course this (up or down) is dicataed by angle of the shot and whether the deer is alert or relaxed.