Start teaching this exercise with your dog on leash.  Toss a treat so the dog has to go to the end of the leash to get it.  As soon as the dog picks up the treat back away from him and say, "Fido, come" using a very happy voice.  When your dog comes to you hand him another treat.  Repeat this, repeat this, and repeat this.

Keep your voice happy and make this fun and exciting for your dog. 

Very quickly you will graduate to working on this outside.  Instead of the leash attach a long line to your dog.  I like to use a 30-foot cotton lunge line for horses to save my hands but a nylon rope and a pair of gloves work well too. 

The keys to having a dog that always comes when called is to make it fun and make yourself a higher reward for the dog than anything else.  It can become quite challenging to teach your dog to ignore other dogs, cats, birds, kids, cars, etc. and only pay attention to you.  That is why you have him on a long line.  Then, when your dog chooses to ignore you in favor of investigating something that is more interesting, you have the tools to correct him and make him come to you for his reward.  Of course, praise, praise and more praise the entire time your dog is running back to you.  Then reward with a yummy treat or two or three.

If your dog happens to get loose, don't chase him.  Dogs quickly discover that this is a great game -- a game of high rewards to the dog.  Instead, run the other direction to get your dog to chase you.  If he won't let you get your hands on him, toss a treat in his direction and again run the other way.  Keep doing this until your dog figures out that you have good things and you will be able to catch him.   I always like to have hold of the dog's collar before I give the treat.  This prevents him from learning to get the treat and then dashing off for more fun.

It is imperative to remain calm and not loose your temper even if this takes hours, and the first time it might.  When you have once again gained control of your dog, don't punish him.  Instead praise, praise, praise and give lots and lots of treats. 

If you punish your dog when you finally do catch him, you are teaching him not to come to you.  You are teaching him that he will be punished when you catch him so he better stay away from you.  Dogs remember the last thing that happened.  When dogs are punished after they are caught, they don't translate that information into, "Gee, I got in trouble for running off.  I won't do that again."  Instead they think, "Wow, I was having fun running all over but as soon as I was caught I got yelled at and punished.  I't must be a bad thing to come to my person." 

Dogs that have been running away from their owners or previous owners for some time require a lot of patience to get them over it.  Each time it will take less and less time but soon the dog will stop running off from you as you have now taught him that the best things for dogs happen right by your side.