Best way to meet a dog you don't know
There is an old theory that when you meet a dog you do not know, you should stick out your hand, palm down and let him sniff the back of your hand.
This is in fact a bad practice. By sticking out your hand toward a dog who does not know you, and who may be very wary or even frightened, you threaten him by invading his space. He cannot be sure, when your hand comes toward him, that you do not intend to grab him.
A much better practice is to stand normally, let your hand lid against your thigh, and then tap your fingers against your leg, making your hand behave like a dog's tail wagging. As you do this, speak reassuringly to the dog. Let him come to you rather than you going to him. If you continue to "wag your tail," most dogs will recognize the gesture as a sign of friendliness, and some even will wag their own tails in response.
Make no attempt to go to the dog until he has shown desire to meet you first.
Do not stoop down or sit to be "at his level." This creates a challenging situation to a dog. Do not stare into his eyes, for the same reason. Look down and to the side.