First off, it is better for your dog to always feel happy to return to you. If you call your dog back only to punish him, he will form the impression that you are going to punish him whenever he answers to your whistle recall command. So be nice to your dog each time he answers to your recall command. Praising and rewarding form the best known method for getting him to appreciate that you appreciate him.
To make your dog leave whatever it is engaged in and run back to you whenever you call is successful dog obedience. Though, this can only happen if you make him feel that it is a good thing to answer you quickly each time you call. Dogs are reasonable animals, your dog wants to be certain that whatever move it makes is beneficial.
Experts advice that you make your dog sit at a distance and then step away from him, keep some distance between you and then combine voice and whistle by saying “come” and blowing the whistle immediately. Do not start with very long distances, rather, increase the distance as he begins to associate the whistle with a recall.
Show him that you are pleased that he answered your call quickly; make some happy body movements like clapping your hands etc. Remember to use a high pitch voice when calling him to return, if you are not using the whistle. But ensure that your call does not sound threatening.
In teaching your dog the recall commands with the whistle, experts advise that you make him sit at a distance and then try recalling him with the whistle. Now, you do not have to start with very long distances, rather, increase short distances as he learns to respond to your recall. It is a proven fact that dogs obey better from a sitting position, so make your dog sit while imparting each new command, or a command that is difficult for it to grasp.
All said, distance recall training is very important because you should be in a position to recall your dog from whatever it is engaged in, and from a distance. However, realize that you may not be able to achieve success as quickly as you desire, but persistent uniformity of commands will surely get your dog to eventually learn to react positively to your whistle recalls from long distances.