While the enthusiasm a dog displays when he first meets someone is largely why they're considered man's best friend, many dogs can develop rude and disruptive behaviours in terms of how they show their affection. Chief among these behavioural shortfalls is the habit of jumping up at people. Even if their owner finds this charming or funny, it can annoy, frighten, or even injure others. As such, teaching your dog that it's impolite to jump at people is an important part of its proper domestication.
A dog cannot be taught to make situational behaviour distinctions, so jumping should never be encouraged if the training is to be effective. Many owners make the mistake of pushing the dog away, and may yell, but much to their frustration, the animal never seems to get the message. But consider the interaction from the dog's point of view. Imagine your frustration if someone seemed to be inviting you to play, but then became angry with you half the time, simply for taking up the invitation. Your own excitement looks very much like play to him. Therefore, the best approach to stop a dog jumping up is to deny him the reward he's looking for: attention.
Before beginning anything else, the dog must be a master of the sit command. This helps at home, but more importantly, when dealing with strangers outside or guests at home. Teaching him that sitting is the quickest path to positive attention is the foundation of polite behaviour. Additionally, learning to maintain a solid sit will teach your dog how to manage his excitement.
The next step to is to calmly ignore the jumping dog. When he comes to greet you, if he jumps, cross your arms, snap your gaze up and away from him, and turn away a step. Keep up with this until he gets the message that he won't get attention by jumping up. A dog already trained in sit will usually try that behaviour next when he realizes jumping is getting him nowhere. Reward the sit with enthusiasm, but resume ignoring him if he begins to jump again.
If your dog is crate trained, try crating him whenever he jumps, only letting him out when he calms down. Any attention given to him during this process must be kept as minimal as possible. It's not a punishment, but providing him the space to calm himself.
Remember, it's important to not get discouraged if results aren't immediate. With enough patience and consistency any dog can unlearn this habit. You will find lots of forums and online discussions where you can post a question to solicit some form of advice of feedback. If however still find yourself having problems take a look in the local newspaper where many dog behaviour experts will be offering their services. While you might not find the next Dog Whisperer, in most cases it’s just a question of having an expert on hand to give your the needed push necessary to change the dogs behaviour.