If your dog tends to jump up on people, it's because he's happy to see them and he wants some attention. This much is reasonably obvious. Dogs are not generally known for their subtlety.
Lots of people say that thunking a pooch in the chest with a well-placed knee when he jumps is the way to go, and that may work for some critters. But it's dangerous. Some dogs have had ribs broken during this training exercise, and besides...it's not very nice to inflict physical pain on someone who's just saying hello. (No matter how obnoxious he's being about it.)
Other people will tell you that a sharp, loud "NO!" or "OFF!" will help deter him from jumping, but guess what...more often than not, this method of verbal correction will exacerbate the problem. If your dog is jumping for attention and he gets any kind of attention at all, the behavior is working for him. In other words, you may accidentally be reinforcing Fido's jumping in your efforts to correct it.
So what's the best method? Swift application of the LRS, or Least Reinforcing Stimulus. All species of animals (including, and probably especially humans) repeat the behaviors that are the most and most consistently rewarding. Removal of all types of reinforcement in a religiously consistent manner when your dog jumps is the swiftest and most reliable way to teach him to keep his muddy little feet on the ground.
The "two-man" method is my favorite:
  1. One person stands still while holding a leash that's connected to a dog. (Preferrably the dog you're interested in training.) This person's job is to stand stock-still and say nothing at all, while holding the leash at a reasonably short length.
  2. Person #2, equipped with tiny pieces of dehydrated chicken liver or little bitty hunks of cheese, approaches the dog from about 15 feet away.
  3. If the dog jumps on the approaching human, said human immediately turns his back and leaves, without vocalizing, without making further eye contact with the dog, and absolutely without touching him.
  4. Step c should be repeated until the dog does not jump. For some dogs, two or three repititions will do the trick, but for others, this could take a while. Be patient.
  5. The very first time the dog does not jump when approached, the approacher should offer him one of the tiny food reinforcements he's hiding, pat the dog on the head, tell him he's a genius, and then walk away again and re-repeat the approach.
  6. From this point on for the rest of the training session, every time the dog refrains from jumping, he should be swiftly reinforced with food and praise. Every time he does jump, he should be swiftly and carefully ignored...(in other words, the aforementioned LRS should be applied.)
  7. Your training sessions should be no longer than 5 to 7 minutes, especially in the beginning. If you work past your dog's attention span, he will be too bored to enjoy the exercise and he will end up dreading it the next time you try to practice.
  8. Once you get 6 to 8 successful, jumpless approaches, stop practicing for the time being. (As your dog begins to master this exercise, you can make your sessions a little longer and begin to require more successful repititions before quitting time.) Always end your practice sessions on a high note to help ensure that your dog will anticipate the next session with tail wagging enthusiasm.
If the same person is always and invariably the person who plays the "approacher" role, it is possible that your dog will effectively learn not to jump on that person in particular, so it's a good idea to enlist the help of all the members of your household and whatever friends you might have that'd be willing to play the game.
During your dog's regular life, when he's not on a leash and being specifically schooled on this matter, and he jumps to greet someone, that person should absolutely withhold ALL forms of attention and reinforcement by either turning his or her back on the dog and refusing to acknowledge him, or by walking away entirely. Yes, this is a pain in the butt, but it is also extremely effective, because it gives the dog an opportunity to choose his own alternate behavior, which leads him to believe that NOT jumping on humans has been his own glorious and rewarding idea. If a dog believes he has come up with a reinforcing behavior on his own, the simple act of exhibiting that behavior becomes a reinforcement in and of itself, which is really nifty, because it means you won't have to keep re- and re-teaching him the same thing over and over.
As your dog begins to learn that there's no payoff of any kind for jumping, he will decide more and more frequently not to bother with it. Eventually not jumping will become a habit, and you won't need to carry liver and cheese in your pockets anymore. (Though intermittently reinforcing a desired behavior with food is a good way to keep it perfect throughout your dog's lifetime.)