RE: Killing distance
Know your limitations and stick with them. It might hurt your feelings to let one pass, but it's a whole lot worse to make a bad shot.
I practice shots at distances that are a lot further than I will take on game. Being a good shot at 40 yds makes 20 and under easier. Practicing at 80-100 yds will really show you how consistent you are and is a good exercise for working on your form.
I've been tempted to take a long shot on a live animal, but I've never done it. Even if I'm feeling good and know I could hit a target at that range, the animal doesn't know it's supposed to stand still until the arrow gets there.
Chad