Watching one lady (who shall remain nameless for obvious reasons), she used so many different blows on her whistle, her dog was clearly confused. Often the lady ended up calling the dog in by name.
If you are one of those people who don't have whistle experience, consider getting out in your garden without your dog and just practice for a bit. Imagine you have a dog with you and commend it appropriately using the whistle. Once you've got the hang of it, introduce your dog to it.
The whistle commands I'm using and seem pretty standard are:
One peep = 'Stop'
Three peeps = 'Come'
Two peeps (peep-peeeeeep) = 'It's there' (use when dog is close to prey/dummy).
I use the whistle loudly when the dog is at a distance, more softly when the dog is nearby.