Most dogs like to jump up on people. This habit is hard to break, particularly when others don't enforce it regularly. I'm not sure if the visitors I get are just too polite or just love dogs, but when I try to get Tilly "off" them, they tend to say, "it's okay."
But really, it isn't okay. She's been living here for nearly two years and acts rather uncivilized STILL!
The first challenge to keeping your dog "off" someone is to choose the word. If you are going to use "down," then "down" is for not jumping, and not laying down. Using "down" for wanting the dog to lay "down" and also for not jumping is too confusing.
The next challenge is being consistent, in all situations. Anticipating when the dog is going to jump is important, and put the dog on a leash so you can reinforce the command.
Everyone on board - if you want to really break the habit, everyone needs to know the "rules" and even if they say it's okay, you will need to explain that you're trying to be consistent with the dog. Otherwise, you will end up with situations like this:
Look how happy Tilly is! And Jane (hi Jane!). How can you say no to that?!