I've called in a total of 3 bears while coyote hunting. I was not trying to call in a bear, but they came in anyway.
I would use a Fawn-Deer-in-Distress call, and keep calling for extended periods of time. If I take a break to catch my breath, it would only be for a minute or 2. Stay on stand for at least 15 minutes.
Also, take a bear cub in distress call with you. You can find them at All Predator Calls.com.
I would also try and call from an open spot, so I could see them approaching from a distance, say at least 75-100yds. The first two bears that came in, I was calling from the edge of the forest and when I first saw them, they were within 40-50yds, and moving in quickly.
Good luck.