I also prefer to aim at the lower third of the bow where the lungs are... A lot of times, a deer will drop at the sound of the shot to coil its legs in preparation for running away. If you aim low, you'll still solidly catch the lungs. If he doesn't drop, you still catch the bottom of the lungs and/or the heart.

Another tip is to bring your sight picture into view from the bottom of the deer, as opposed to the top. A lot of newer archers get jumpy and as soon as brown enters the sight picture, they squeeze off a shot -- resulting in a miss if the deer drops or a high hit, which is not good for a variety of reasons.