One of the most valuable lessons your dog can learn from you is to "leave it." Dogs are naturally curious, and they have the instinct to scavenge food. Scraps lefts on park benches or in the grass have the potential to harm your dog, especially if people have specifically targeted the "leftovers" to be harmful to animals. Other normal people foods and objects can also be hazardous to your dog's health (pieces of leather, chocolate, or raisins, to name a few). Teaching your dog to ignore food that doesn't come from you and to "leave it" when it looks interesting is therefore a needed skill.
To teach your dog to leave it, begin you training session with a dog treat in one hand. Your dog will notice and be excited, sniffing at your closed hand. When he sniffs, say, "Leave it," in a firm voice. Your dog may not immediately understand, but as soon as he looks away from your hand or pulls back, reward him with a different treat using your other hand—the one he was not sniffing. If you do not use treats to train your dog, keep cheese or a piece of leftover chicken in your hand, and reward with a belly rub or another type of positive reinforcement.
To elevate this first training step, put the food in your open palm. When he sniffs, say, "Leave it." If he tries to take the treat, close your hand. Open your hand and say the command again. When he finally turns away, reward him again with a different treat from your other hand.
The next step is to train your dog to leave it when you are not holding the treat. Instead, place it on the floor near you. When your dog approaches, repeat "Leave it." If he tries to eat the treat, cover it with your hand and repeat your command again. Once he walks away or ignores it, reward him.
Now it is time to test the training. Place your dog on a leash and lay some desirable food or toy on the sidewalk. When he sniffs the toy, state your command. Remain standing. If he chooses not to obey, cover the food or toy with your foot. Only after he fully ignores the item on the sidewalk, reward him. After your dog has mastered this command, try the same process when he is off the leash at the dog park. Eventually, your dog will ignore any foreign item he finds on command.
For more pet obedience training tips, contact a company like Petrichor K9.