Why Dogs Jump Up
Dogs jump on people because they have been reinforced to do so. When they jump up, they get attention. However, they need to be taught that in order to get attention, they need to be in a sitting position.

How to Correct Jumping Up Behavior
As with most behavior problems, preventing/redirecting the unwanted behavior while encouraging better behavior is an effective plan.

• Practice "sit". Have your dog sit for everything he enjoys in life from food to petting, from opening a door to throwing a toy. Soon your dog will learn to associate sitting with everything good and this will give him something else to do other than jump up.
  
• Don’t reward jumping up. If you pet, hug or respond excitedly when your dog jumps up, he’ll think that you like it. When your dog jumps up, step forward toward your dog and say ‘Off!’ in a low tone. (‘Off’ is the preferred command since ‘Down’ is usually used for lie down.) If this does not work, consider turning away from the dog. Every time he jumps on you or motions as if he about to jump up, turn and move in the opposite direction.
  
• If he puts all four paws on the floor, praise him calmly. Now direct him to “sit” and calmly praise him again. A kibble of food would be a good reward as well. If he does not respond, try luring him off you with a treat – preferably a piece of his food – and into a sit. Practicing this can quickly give him an acceptable way to greet you.
    

For Your Guests
Consistency is key to training a dog. Stand next to your guest and practice your greeting routine with your dog. Practice with dog-loving friends who understand that training takes time. If this isn’t possible with some guests, then prevent the issue entirely by confining your dog when your guests arrive. You can always bring him out later when things are calmer.