I try to think of pointing at the spot with my index finger knuckle (sometimes I do it with my index finger itself) it helps me concentrate. Concentration or focus(along with lots of practice) is the key to instinctive shooting, but you shouldn't have to concentrate on a spot for 30 seconds. I pick a spot, focus on it, draw, focus a little harder, and then release all in a couple of seconds. Along with Bodorks suggestion maybe you could try shooting at moving targets.Moving targets (imo) help you to concentrate and don't give you time to second guess yourself.
Also, I have found that the old adage "aim small miss small" is right on the money. Now, if I could just remember that on every shot. I don't know if you have watched the DVD "Masters Of The Barebow" , but Dean Torges said something in the bonus section at the end with all the bloopers and such. He said iirc " lets see how close I can get to that spot without hitting it" that was really profound if you think about it.