Great advice above,
How high is your stand, a common problem I used to have, shooting from a 30' high stand was the arrow entering the animal at too steep an angle because I was shooting while they were too close. This led to a "one lung" shot and the deer go forever with that type of shot, its important to penetrate both lungs.
I just worked at being more patient and letting the deer get a little farther out from my stand.
As Buckshot said a rangefinder is a great tool to have. I never go out without mine. Once in my stand I use it to mark off the distance of different landmarks around me, a tree, stump, weed clump whatever.
If I am then lucky enough to have a deer come in I already have that distance memorized, for instance "he's right next to that stump which read 26 yds.".
That way I am not fumbling around with the finder figuring distance but already have my bow drawn and am concentrating on proper pin sighting.