When a dog jumps up on people, it is because he or she wants to be at eye level with them. Start by lowering yourself to his level, and reward him at that point, while he's sitting, or rolling, or whatever it is he may do when you get there.
If this doesn't work, a gentle slap to the top of the head will usually make a dog react by either sitting or backing away. The trick at this point is that you have to be very quick with the reward while he's down, because chances are, he's going to jump right back up.
Turning your back on a dog is actually an invitation to follow, so you are not giving him the right body language to communicate you want him or her OFF you! I have had dogs either jump on my back (and sometimes try to hump) or run along behind me scratching at my legs! Spend a little time observing dogs when they greet each other, and if there is a size difference, you will notice they move until they are initially eye to eye.
This training takes patience, it doesn't work right away and it is a behavior that will reappear randomly throughout your relationship with your dog. A happy dog likes to jump and how you train him to stay off should start with the gentlest approach and then gradually escalate to more punitive measures as your dog needs it. It will also need to be repeated periodically until your dog has aged enough to settle down.
Sometimes, you do have to enlist a partner to hold your dog on leash when he is approached.
One option that I like to use, is to teach your dog to jump up on you ON COMMAND. This allows him that moment with you on your terms. This is VERY easy to train a dog to do.
Choose a command that is very specific and can't be confused with other command, general conversation, or body language. You wouldn't want him to misunderstand and jump on someone randomly in a group because they say or do something he thinks means to jump on them! Treats and other rewards most likely won't be necessary, because being able to jump up on you is the reward itself to your dog!
As a note: A dog's nostrils run along the top of the muzzle and this area is a fairly delicate anatomical part, not to mention crucial. It is better to tap your dog on his or her skull, which is a very thick, protective layer of bone.
Good Luck and Good Dog!