This is a common issue pet parents face, mostly with young dogs and puppies, especially when the weather is pleasant and more and more people want to say hi to your pooch while on walks!
But what can you do to best tackle your dog's constant jumping on people, and even better, how can you make sure you prevent it to the point it becomes a behaviour your dog no longer finds purposeful? (Because yes, your dog is doing it for a reason!) Let's start with what the reason behind this behaviour might be. The most common motive behind your dog jumping on other people may be two-fold:
1. It works!
Like all other behaviours your dog does, it's mainly because it pays off for him and that can be in a number of ways. The main way in which jumping on other pays off is in what they get in return which is attention, whether it comes from you (even though you're not trying to praise or reward them, your attention is) or from the person they're jumping on.
It's rather difficult to expect of the person getting jumped on to not react and give your dog any kind of attention, in order to avoid reinforcing the unwanted behaviour, especially when your dog is coming off friendly and/or when the person in question doesn't know that this is a behaviour you're working on managing. A tool that could help would be a label or sleeve that goes on your dog's leash or harness that lets others know to not pet or engage with your dog, or that he/she is in training.
But something more that you can do in this case to yield far better results is to go for the preventative approach, to avoid having to manage a scenario that is largely not under your control and that you and your dog aren't fully ready for anyway. The best way you will prevent your dog from jumping and getting reinforced by other people is to become familiar with the body language patterns and signs that let you know that your dog is going to be jumping on them so that you can right away do your best to engage your dog with a different activity/behaviour, that you like better instead. Such as walking away and tugging on a toy with you, fetching a ball, or walking away with you and tossing treats for them to search through the grass or under a chair if you're indoors. Prevention is key but will only work if you're in tune with your dog's demeanour and patterns so that you can try to prevent them from doing the unwanted behaviour altogether. The more they get to practice it, the more they are likely to repeat it. The more you find yourselves ready with a leash and treats/reinforcers ready, the more successful you will be. These are the best ways you can manage greetings with minimal jumping on strangers, while you practice working on your dog's greeting with people in your close circle that will be able to help you in responding accordingly, should/when your dog happens to jump on them. More on this on the following point.
2. Your dog has a very social nature and/or is a high-energy dog!
You can't train a dog's personality and demeanour out of them, but you can definitely find ways to get what you want if you also focus your efforts on ensuring your dog gets what he/she needs on a regular basis.
If you live with a social AND energetic dog, you will want to make sure that you provide him with regular outlets for their need to play, jump, be excited, and be friendly, otherwise, they will find those outlets on their own and can result in more jumping on others, or other unwanted behaviours. Great outlets consist of dog training classes of different types, supervised play sessions, agility training, and other controller enrichment/engaging settings. And don't worry, your dog getting to practice jumping in a setting like play sessions, for example, can be different and not at all encouraging further jumping outside of those environments because dogs have a very strong capability to associate behaviours to places/scenarios we put together. This is done by rewarding your dog for being themselves mostly and mainly in that particular setting, but not as much (if at all) outside of it. Just like how you know that you can act a different way at a restaurant and another way at a concert.
But then what follows that? What follows your dog having the outlets he/she needs? You. You and the environments you create where you are bringing home a happy, fulfilled, tired do that is less likely to feel the need to jump on the next stranger you encounter on that evening walk. But that's only part of the solution, the other part (especially when you've just begun tackling this behaviour) is having the environments you work in tandem with where you and your dog are at. This means that instead of practicing walking your dog past strangers in the hopes that he doesn't jump on them, you'll want to practice with people that know you and your dog, so that you can ahead of time let them know that if ever your dog jumps at them, that you would appreciate it if they did their best to not respond to your dog's jumping until/unless he's saying hello to them without. The combination of a dog that doesn't feel as much of a need to jump on people, along with an environment that doesn't put much (or any), attention to the unwanted behaviour WHILE ALSO rewarding and acknowledging the moment your dog does make the choice to not jump or to stop jumping, is what will help you and your dog succeed!
If ever you're finding that your dog might be jumping on other people, but even more so you, then you may want to consider what your dog is trying to communicate to you there and then. This is because jumping on you or another pet parent can be a way of communicating that they're feeling overwhelmed, tired, overstimulated or even anxious at that moment, or about the environment they're in. This would require a closer look at when it's happening as adjusting the environments in question and our expectations is what will also help you in your success as well.