I learned this years ago, as did many of my colleagues in the world of guiding Maine moose hunters. If you call in a good bull and don't kill him, your chances of calling him in again the next day, or in the next month, are very slim. You may think moose are stupid, but if you do this, the joke's on you. Furthermore, you have a pretty good chance of calling him in without actually ever knowing it, but he's been educated all the same.

Practice your calling at home, and save your in-the-field calling for when you are actually hunting.  It's not rocket science, a car horn, or one of my old Janis Joplin records will work. Good luck!