When you are attempting to train your dog to "come" on command, it is important to train non-emotionally (i.e., no yelling or screaming). The best method is to "load" a sound using treats or a toy of some sort. Basically, all this means is that I am going to associate a sound with giving my dog a nice treat. For example, I make a sound like "WooHoo" and immediately give my dog the treat. The dog then starts to associate that sound with getting a treat and essentially will respond accordingly.
You can use a variety of different ways to teach your dog to come on command using a variety of different sounds. I have personally found whistling to be one of the best sounds to use in training a dog to come. The thing to keep in mind here is that it should be a sound that is easily heard when around lots of commotion and possibly other dogs such as in a busy park.
When you are dealing with distractions like playing with other dogs, you should make sure that the sound you choose will work before you start repeating the sound. It should not blend into the background noise because it will be ineffective. The best way to associate or load the sound for your dog is to do it at home and then move to a park that doesn't have a lot of distractions before going to the busier places.
Once you have your sound loaded and you are confident your dog is responding to it, start playing games with your dog. You can do something like throwing a toy. Try faking your pooch out with a throw, and as your dog starts after it and realizes the toy isn't there, it will naturally run back. But as it turns to run back, make the sound you loaded and then give your pup the treat. This begins to reinforce the sound with the treat and will get your dog to respond appropriately.
One of the best games to play with your dog and get them to know where you are at all times is hide-and-seek. It's best if you can have someone hold the dog while you go hide. Once you are hidden, have the person release the dog and you should begin making your sound. Once the dog locates you, give it the treat and give it praise. It's a fun game to play, and it teaches your dog to find you instead of you searching all over for it.
So, loading sounds and keeping it non-emotional is one of the best ways to teach your dog to come when you call it. You can also do things like load its name instead of a generic sound or whistle. A dog's favorite game is chase, so play it often with your dog while incorporating these techniques and soon your dog will be responding just the way you want. Always remember, repetition and reward are the keys to successfully training a dog.