The good Sisters of Saint Joseph did not teach me how to whistle, spit or swear. I have managed, over time to pick up the last two, but now need to whistle to direct Dusty at a distance.
I have seen people using the shepherd's whistle, but Connie Brannen's account - "I remember the first time I put a shepherd's whistle in my mouth--a silent little bubble dribbled out one side while my eyes nearly popped from the unreleased pressure behind the instrument." - makes me think I should try other methods first.
In The Other End of the Leash, Patricia McConnell says that universally people working with aninmals use a three-short-sounds call, so this has been our starting point. I've been practicing a three note whee, whee, whee as a recall. The Commander whistle people provide some useful sound files to copy.