I know this is a little late but better late then never. Whistles are an important part of working together. They help the hanfler as much as the dog. My dogs don't like to be hollared at no matter how far away they are so the whistle is the ideal way of giving directions.
The best way of becoming proficient with whistle is practice. I use to have one in my mouth anytime I drove anywhere. Drove my wife crazy ... it must have worked now I go where I'm going with just me and the dogs:)
When I am helping folks train their dogs I insist that when first starting out they keep there hands in their pockets. The last thing I want is the dog looking back for instructions. If they are doing that they are not looking at what they are suppose to be doing.
I have been writting a training blog aimed at the working dog rather then the trialling dog. It may have some use to you although I have not tackled whistles yet http://workingstockdog.blogspot.com/
Practice practice practice and oh by the way ... better have a rag with you ... it gets kinda messy at first