i live in northwestern montana, aka the largest population of grizzly bears in the lower 48. So I've had a bunch of experience running in bear territory.
For the most part, bears want absolutely nothing to do with humans. They're terrified of us and the irrationality we runners present to them as we're running up the trail. Behind my house, I've run into bears about a dozen times. Every single time that I have seen them, including twice with cubs, the bears have taken off in the other direction as hard as they possibly could.
It still shook me up and I don't take these experiences lightly. However, it's a great experience to not feel like the top dog in the world. It's a feeling that causes immense respect, amazement, and fear.
So i carry bear spray, run away from the smells of dead carcasses, and make noise when i see a large scat. Otherwise, the other bear tips someone gave are great. However, don't stare the bear in the eyes. Yes, face them, but look down away from them so that they know that you know the bear is present but don't want to fight.