ive got a spot right near a dam and all these suggestions are good. make sure you dont shoot the roost but set up along the way right between. one of my favorite spots is right by a roost but far enough away that they dont disappear for the season, but close enough that they are on their way to land and not 500 feet up.
dont let them see you on your way in or out, or they will get the hint and take off somewhere else. also try not to hunt it often. once every few weeks and hope that someone else doesn't hunt the spot.