My dog jumps up. How can I stop him?

Many dogs are inadvertently rewarded with their owner's attention and eye contact when they jump to greet.

The most basic law of learning is that behaviours that get rewarded are likely to be repeated. This applies equally to both good behaviours and bad behaviours.

Withdrawal of ALL attention [including the reward of paying attention & pushing the dog away and speaking to the dog by saying 'No'] whenever the dog jumps will ultimately lead to the behaviour disappearing and becoming extinct. 
It is physically impossible for a dog to jump up and sit at the same time! This is why teaching dog to sit instantly or go into a drop (down) position is by far and away the quickest and easiest method of preventing a dog from even thinking of becoming semi airborne.
Teaching a dog to 'drop' on request is as easy as 'sit' and the steps, which should be taken gradually, are as below:
How to teach Drop - not Drop, Drop, DROP!
Your dog already knows how to drop (or down) and does so all the time by himself! Your job is to get your dog to drop by use of either a verbal or a hand signal, whenever you want him to do so.
 A reliable drop (first time, and remain dropped till released or told to do something else such as sit, stand or come is the second thing that you should teach your dog to do. Why?
    • A dropped dog cannot pull on lead, chase things, dig, escape or bite other dogs or people. It is hard to bark when lying down.
    • It is the second easiest of all behaviours to train: you and your dog will both have immediate success.
There are several ways of teaching drop. The traditional method is to say 'drop' while at the same time using physical pressure and pushing the dog down or pulling its legs from underneath and then saying 'Good dog' as it collapses into position.
 Teaching drop with modern reward-based training is equally effective and, if done correctly, the dog, learns the behaviour quickly, understands it and remembers it for all time.
 The up-to-date method for teaching an instant drop consists of the following steps:
  1. Get the drop by a) either waiting for the dog to drop of its own accord or b) get your dog to sit and then lure it into position with a small piece of food. Move your hand slowly downwards from the dogs nose to the floor the eyes and head will go downwards and, inevitably, the front legs will fold into a drop. Alternatively, sit on the floor and lure your small dog under your bent knees.
  1. Mark the exact moment of contact (stomach hitting floor) with a 'That's right' signal such as an excited 'Yesss!' a click with your fingers - or by using a clicker.
  1. After the mark, immediately reward the behaviour (stomach hitting floor) with the piece of food.
    1. Practice the drop hand action at least 10 times a day for three days WITHOUT using any words. Your hand action, without food in the hand, soon becomes the hand signal for drop.
  1. Start phasing out the use of food. Only reward every second drop, then every 3rd drop and so on until the reward comes as an unexpected surprise on a random basis.
  1.  Reward from the other hand, not the one initially used to hold the lure. Start keeping the food somewhere else, not on your body, as on a plate or dish.
  1. On the fourth day, start saying 'drop' quietly AS the dog is going into the drop position.
  1. Once the dogs definitely associates 'Drop' with the action of dropping then, and ONLY then, start introducing the word beforehand. It will become a meaningful instruction ('drop'), not a shouted and frequently repeated and irrelevant command (Drop, DROP, DROP!)
  1. Practice drops in many different places, inside and outside the house. Sometimes use hand signals only, sometimes use hand and verbal signals, and finally the hardest of all - verbal signals alone.
  1. Increase the length of the drops from 2 seconds to 3 minutes by small daily increments of 3-5 seconds
    1. Gradually start introducing distractions, such as when there is another dog 100 metres away.