Here's some information shamelessly stolen from the Springer site I'm a member of:
Whistle Training
Training your dog to some basic commands in response to the whistle is not complicated if we think that the dog simply sees it as another command that you are introducing.
With both a puppy and older dog food is a great motivator, and when introducing the whistle why not start when you are calling your Springer for its dinner, use its name and follow immediately with 3 pips on the whistle, in no time at all the dog will be rushing to you with just the whistle signal, this is basic positive reinforcement - your Springer realises that to respond to the whistle will always equal a reward.
Now move to your garden, and reward your dog when it responds to the whistle with some form of reward, either a titbit or praise. In no time you will find your dog has picked up the first of your whistle signals.
Secondly, you may wish to start introducing a stop whistle. Call your dog, tell it to sit and then follow the command immediately with a single pip on the whistle, then praise your dog. Tell the dog to stay, take a few steps away and call the dog, trying again, to tell the dog sit, followed immediately with a single pip.
Try this about 5 times, and then on the next go leave out the sit command, call the dog and just give the single pip. It the dog sits then give it lots of praise and a reward, if not give the sit command, followed by the whistle. Don't swamp the dog if it doesn't get it first time, try again later or tomorrow. If your dog sat for the whistle when training the next day still give the verbal command the first few times just to make it clearer to the dog, and hopefully by the end of the week the dog will sit whenever you give a single pip.
http://www.itsaspringthing.net/training/gundog.php