I agree with fishy look at different ducks in different poses and chose what catches your eye right away. Some do look better sitting than flying and vice veresa. It' s all up to you.

A flying mount is definitaly my favorite, but some ducks lend themselves to nice standing mounts. If you have a nicely decked out mallard or wood duck, a standing mount is the best way to display most of the colors and patterns on the bird. If you were to mount it flying you would cover the feathers on the side of the bird. Also, if they' re standing, you can give them a nice big puffy head, and still have it look natural