Hi Liz,
Whistle Training from my point of view!

Do it! It saves on the sore throats! :D This is similar to what others have said, but is based on the advice given to me by a gamekeeper and various gundog books. I hope the instructions are clear.
To teach the dog what the whistle command means, try this:
In the house, whistle held between your teeth, say "
dog's-name COME!", then
pip, pip, pip in quick succession,
immediately the dog starts heading towards you. Timing is important. Do this as often as possible until you are sure that the dog associates the whistle with the recall command. When the dog responds to the recall every time, then you can try without the verbal cue, but still indoors.
When the dog responds quickly to the whistle alone, every time indoors, then you can move into the garden... and finally into open spaces, but don't rush it if you want it to be reliable. :)
Using a whistle is very effective if you keep it simple. Always be consistent with how you blow the whistle. IMO, short sharp pips are easily recognisable to the dog and they give a sense of urgency, so
pip, pip, pip - sounds like "come here quick" in my head - easy to remember. :) Whereas a long
peeeep sounds like shouting "STOP!" and is often used in gundog training for just that, to stop the dog (ie in a sit or down, or even still standing), so that you can give him a further instruction, like a change of direction. However, I would only teach a recall if that is what's important to you. Then there can be no confusion about what the whistle means. :D
From
Turner Richards Website, get an Acme High Pitch Plastic whistle, without a pea, number 210.5 or 211.5, it doesn't matter which. They have slightly different pitches, but if you need to replace it, make sure you get the same pitch again. As for a lanyard, a piece of string or ribbon will do the same job! BTW, the whistle might not sound loud to you (outside, that is!) but the dog will hear it from a long way off. ;)
I hope this helps. Good luck!
Monty's Mum :)