Practice, Practice, Practice
Your dog has probably already had plenty of practice jumping up on people. Now it's your job to make sure he practices not jumping enough to override all the practice he's had, and all the reinforcement he's gotten, from jumping up.
Dogs that learn proper greeting habits from the beginning may never consider jumping up. From the moment you get your dog, make sure she is not reinforced for having her feet on people, and that the only way she'll get attention is by controlling herself, staying calm, and sitting (or whatever you want her to do instead of jumping).
Set Up Training Opportunities
Your dog's new mutually exclusive behavior to replace his default behavior of jumping up won't happen if you wait until you need it to work on it. You absolutely must set up training opportunities to get enough practice in for the new behavior to become the new habit. Have a jumping-up training party if you can, or at least invite (beg or bribe, do what you've gotta do!) a few people a week to come knock on the door for the specific purpose of training. Recruit anyone who happens to knock on the door to help. They don't necessarily have to pet or interact directly with your dog, but maybe they'd be willing to wait patiently while you get your dog into a sit. Maybe they'd even be willing to knock again in a few minutes so you have another chance to work on it.
Seek out real-world training opportunities, too. Take your dog to a pet store, or to your town green or park, and ask anyone that wants to pet your dog help you reinforce the sit or down for greeting. Most people who want to pet your dog will be glad to help you. If they are unwilling or unable to help you, either step on your leash so your dog can't jump, or don't let your dog greet that person.
